Add OpenXR hand tracking demo

xr/openxr_hand_tracking_demo/.gitignore

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+# Ignore our Android build folder, should be installed by user if needed
+android/
+
+# Ignore vendor plugin add on folder, should be installed by user if needed
+addons/godotopenxrvendors/

xr/openxr_hand_tracking_demo/README.md

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+# XR Hand Tracking Demo
+
+This is a demo showing OpenXRs hand tracking and controller tracking logic.
+
+Language: GDScript
+
+Renderer: Compatibility
+
+> Note: this demo requires Godot 4.3 or later
+
+## Screenshots
+
+![Screenshot](screenshots/hand_tracking_demo.png)
+
+## How does it work?
+
+Being able to see the players hands, and having those hands interact with elements in the environment are paramount to a good XR experience.
+
+In this demo we look at the off the shelf logic for displaying a hand model that is automated based on either controller input or through optical tracking of the players hands.
+We also implement logic that allows interaction based on input from the action map that allows the user to pick up the blocks in this demo.
+
+The problem this poses to us is that there have been two schools of thought around what hand tracking actually means,
+and depending on the XR runtime in use there may be gaps in functionality.
+
+### Hand tracking is only for optical tracking
+
+The first school of thought treats hand tracking as a separate system that purely focusses on optical hand tracking.
+The hand tracking API in OpenXR, even if reported as supported, will only provide data if optical hand tracking is used.
+
+This means that when controllers are used, no data is available and you as a developer have to come up with your own
+solution for displaying a hand mesh and animating it according to controller input.
+Note that the current version of Godot XR Tools contains a full solution for this.
+
+Equally in this line of thought, the action map is only applicable to controller input.
+You as a developer are responsible for implementing some means of gesture recognition when optical hand tracking is active.
+
+This becomes extra nightmarish when support for both controller tracking and optical hand tracking needs to be supported in a single application.
+
+### The unified approach
+
+The second school of thought ignores the differences between controller tracking and optical hand tracking
+and treats them as two versions of the same.
+Especially with controllers like the Valve Index, or with various data gloves that are treated as controllers,
+there is no discernible difference here.
+
+The hand tracking API is mostly used for visualising the players hand with bone positions either being inferred
+from controller input or matching the optical tracking.
+For advanced gesture recognition you would still use this data however it is now accessible regardless of
+the physical means in which this data is obtained.
+
+At the same time, in this school of thought the action map system is seen as the primary means to gain input
+and is no longer restriced to input from controllers. The XR runtime is now responsible for recognising base
+gestures such as pinching and pointing resulting in inputs that can be bound in the action map.
+
+OpenXR is moving towards this approach and this demo has been build in accordance with this however not all runtimes have been updated yet.
+
+SteamVR has followed this approach for a long time and works out of the box, however SteamVR treats everything as controllers resulting in some short comings when a Quest is used over Meta Link or Steam Link and optical hand tracking is used.
+
+Metas native Quest runtime on all versions of Quest now support OpenXRs "data source extension" which Godot enables when hand tracking is enabled.
+However Meta does not yet support OpenXRs "hand interaction profile extension" which is required.
+
+Meta link is still trailing behind and does not support this brave new world **yet**.
+
+For other runtimes like Picos, HTC, Varjos, Magic Leaps, etc. may or may not yet support the required extensions.
+
+### Conclusion
+
+Due to the wildgrowth in capabilities in XR runtimes,
+and there being no solid way to detect the full limitations of the platform you are currently on,
+Godot XR Tools does not have support for the hand tracking API and purely relies on its own inferred hand positioning approach.
+
+However with more and more runtimes adopting these new extensions any solution that targets platforms with support,
+it is becoming possible to rely on the hand tracking API.
+
+This demo project shows what that future looks like.
+
+## Hand tracking API
+
+As mentioned, the hand tracking API is at the center of visualising the users hand.
+In Godot 4.3 we overhauled the system so the XR Interface needs to convert hand tracking data to the Godot humanoid skeleton hand bone layout.
+This also means that this logic works both in WebXR, OpenXR and any other XR Interface that adds support for this feature.
+
+Hand tracking now also makes use of the new Skeleton Modifier logic in Godot 4.3 however
+the skeleton is posed in the hands local space, while positioning is provided through a XRNode3D node.
+
+This split is applied because:
+
+* positioning is always within the local space of the XROrigin3D node
+* there are many use cases where the positioning may be ignored or modified
+
+> Note that the trackers used for the hand tracking API are `/user/hand_tracker/left` and `/user/hand_tracker/right`.
+
+## (Half) body Tracking API
+
+Just an honerable mention of this, this is not part of this demo but Godot now also has support
+for half and full body tracking that includes hand tracking. This functionality however is only
+available on a limited number of XR runtimes.
+
+## Action map
+
+As mentioned, we're using the action map here for input however when optical hand tracking is used
+we rely on OpenXRs hand interaction profile extension. Without support for this extension this demo
+will not fully function.
+
+This can be solved by checking that no interaction profile has been bound to our XRController3D node,
+and performing our own gesture detection.
+As this would greatly increase the complexity of this demo and the expectation is that this extension
+will soon see wide adoption, this is left out of the demo.
+
+> Some headsets will support the simple controller when hand tracking is enabled.
+> The simple controller interaction profile doesn't support an input for grabbing.
+> In this scenario you can grab the cubes using the pinch gesture
+> (touch the tip of your thumb with the tip of our index finger).
+
+We are not using the default action map and instead have created an action map specific to this use case.
+
+There are only two actions needed for this example:
+- `pose` is used to position the XRController3D nodes and mapped to the grip pose in most cases
+- `pickup` is used as the input for picking up an object, and mapped accordingly.
+
+The pickup logic itself is split into two components:
+
+* `pickup_handler.gd/.tscn` is an Area3D node with logic that is added as a child to an XRController3D node and handles the logic for that hand to pick up objects in range.
+* `pickup_able_body.gd` is a script that can be added to a RigidBody3D node to make it possible to pick up/drop that object.
+
+> Note that the trackers used by the action map are `left_hand` and `right_hand`.
+
+## Local floor reference space
+
+A final notable element is that this demo uses the local floor reference space.
+
+With this reference space the XR runtime will center the player on the XROrigin3D node when the user triggers the recenter logic.
+The startup behavior is different between different XR runtimes, Quest will attempt to remember where you recentered last, while SteamVR tends to reset this to default.
+It can thus not be guaranteed the player is in the correct spot when the demo starts.
+Hence the instructions suggesting the user recenters.
+
+## Running on PCVR
+
+This project can be run as normal for PCVR. Ensure that an OpenXR runtime has been installed.
+This project has been tested with the Oculus client and SteamVR OpenXR runtimes.
+Note that Godot currently can't run using the WMR OpenXR runtime. Install SteamVR with WMR support.
+
+## Running on standalone VR
+
+You must install the Android build templates and OpenXR loader plugin and configure an export template for your device.
+Please follow [the instructions for deploying on Android in the manual](https://docs.godotengine.org/en/stable/tutorials/xr/deploying_to_android.html).
+