RigidBody center of mass with Bullet/GodotPhysics

[HugoSlt]

As discussed in this issue, it's not obvious what the center of mass of a RigidBody is, especially since Bullet and GodotPhysics have different conventions.

We should add a note in the docs that mentions :

• GodotPhysics defines the center of mass as the average of the collisionShapes centers
• Bullet defines the center of mass as the RigidBody center
• Thus with Bullet we can manually set the center of mass where we want by adjusting the relative positions of the RigidBody and its children
• Optionally, maybe also add a note on how to compute the center of mass of a complex imported mesh. If Godot doesn't have a built-in function, then maybe we could just mention that Blender has the "Origin to Center of Mass (Surface)" and "Origin to Center of Mass (Volume)" functions with shift + ctrl + alt + c ("Set Origin")

[pwab]

What an interesting observation! This could explain some headaches I got while I've worked with RigidBodies and Joints.

Just a quick sidenode:
The information that GodotPhysics defines the center of mass as the average of the collisionShapes centers could also be added to the RigidBody2d docs. This could be mentioned when documenting these functions:

• add_central_force
• apply_central_impulse
• add_torque
• apply_torque_impulse