Proper support for custom mass properties in 2D/3D physics bodies

[pouleyKetchoupp]

Default center of mass in 2D:
Compute center of mass based on shapes by default in 2D, same as in 3D.
This change breaks compatibility because it was always using the center of the body before, although in common cases the computed center of mass is still going to be at the center.

Custom center of mass:
Can be set through the physics server using:
PhysicsServer2D.body_set_param(get_rid(), PhysicsServer2D.BODY_PARAM_CENTER_OF_MASS, center_of_mass)
Can be reset through the physics server using:
PhysicsServer2D.body_reset_mass_properties(get_rid())

3D uses a similar API.

Edit: Now can be set in RigidBody2D and RigidBody3D nodes as well with properties:
center_of_mass_mode: set to Auto or Custom
center_of_mass: custom center of mass (relative to the body's center)

Custom inertia:
Can be set through the physics server using:
PhysicsServer2D.body_set_param(get_rid(), PhysicsServer2D.BODY_PARAM_INERTIA, inertia)
Can be reset through the physics server using:
PhysicsServer2D.body_reset_mass_properties(get_rid())

Can be also set on rigid body nodes through the inertia property, with 0 meaning default calculated inertia.

3D has a similar API, although it's also possible to setup a custom inertia on specific axes, and set the other ones to 0 in order to keep the default computation. That's useful to make a body rotate more or less easily on a specific axis, without affecting other axes.

Full list of changes:
-Added support for custom inertia and center of mass in 3D
-Added support for custom center of mass in 2D
-Calculated center of mass from shapes in 2D (same as in 3D)
-Updated forces and joints to use center of mass in 2D
-Fixed mass properties calculation with disabled shapes in 2D/3D
-Removed first_integration which is not used in 2D and doesn't seem to make a lot of sense (prevents omit_force_integration to work during the first frame)
-Support for custom inertia on different axes for RigidBody3D
-mass property uses a more reasonable slider range by default (max 1000), with the possibility to set higher values

Closes #godotengine/godot-proposals#945
Fixes #12353
Supersedes #29867
Addresses issue from #49185 (review)

[pouleyKetchoupp]

Rebased and added the possibility to set a custom center of mass in RigidBody2D/RigidBody3D nodes.

[Zoomulator]

Could this be cherry-picked to 3.x by setting the default center_of_mass to use Custom, Vector3(0,0,0) to avoid breaking compatibility?

[pouleyKetchoupp]

@Zoomulator Ok, it should be possible but I need to check what can be safely backported as there are changes in the way things are calculated in the physics server itself.

[Zoomulator]

@pouleyKetchoupp I worried that there might be adjacent changes in 4.0.
I'd greatly appreciate it if you gave it a try, if it's not too much trouble! 😄