Add per-Viewport InputEvent device masking

[rjkilpatrick]

Describe the project you are working on:
Split-screen FPS with support for 8 concurrent players split-screen, with up to 8 game controllers at once.

Describe the problem or limitation you are having in your project:
Currently, I have actions in the Input Map corresponding to each game controller incredibly verbosely, e.g. forward_1.
This is hard to maintain and leads to duplicated input map for each player character.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
I propose, one action that triggers across all devices which can be masked per Device, as in the current Input Map.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
Under the current Viewport/Gui settings, there could be added something equivalent to the current rendering targets bit-masking for each physical device.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
It could ben an autoload with a script similar to https://kidscancode.org/blog/2018/07/godot3_splitscreen/ with an export var for the id and string interpolation on the Input.is_action_pressed("forward_%s") in combination with populating the Input Map with code, but in editor would be preferable for consistency with other projects and helping contributors understand where code is located.

Is there a reason why this should be core and not an add-on in the asset library?:
Viewport in editor settings are core.

[cgbeutler]

Yeah, I think they built actions to be emitted to the input callbacks of the tree, not really for polling.

There may be a more general proposal hiding in this one somewhere?

The InputEventAction object passed to the tree's input methods does have a Device field that gets set. The one gotcha that I know of there is that the device id reported is not unique across all devices. If you have Device of 0, that could be a Keyboard, Mouse, or Joypad, so you may have to make those action names unique or something...

Maybe each device could be given a truly unique id, then you could maybe have a call like Input.get_devices_with_pressed_action("forward") that returns a list? or maybe Input.is_action_pressed("forward", uniqueDeviceId)

Anyway, food for thought.

[Sauermann]

The Physical device-Bitmask seems too narrow a solution for the problem, that users want to filter events to be sent to SubViewports on other criteria.

In godotengine/godot#78762 I propose a more general solution to this problem.
That PR introduces a SubViewportContainer-function, that users can overwrite and specify, which events should be sent to the SubViewport.
So for example, you could filter for a Viewport A only those InputEvents, that have a device id 5 and for a Viewport B only those, that have a device id 6.

[djpeach]

To confirm, Sauermann's implementation (split into #78762 and #79480) allows nested input layers/screens without losing focus, local splitscreen menu's, and really whatever the dev wants to do.

Demo