Display the center of mass for RigidBodies in the editor

[Calinou]

See also #945.

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

Depending on how you create the collision shape(s), the RigidBody's center of mass can end up in an unexpected location. There is currently no way to debug this in the editor, or while the project is running with the Debug > Visible Collision Shapes option.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add a way to display the RigidBody center of mass in the editor, at run-time, or ideally both. Being able to view the center of mass at run-time would be useful for those doing procedural generation.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

This can be done by adding a gizmo for RigidBodies in the 2D and 3D editors. To display the center of mass, we can use a bunch of lines similar to the Position2D and Position3D node representations (except with a different color and opacity).

If this enhancement will not be used often, can it be worked around with a few lines of script?

This can be implemented with an editor plugin, but this should probably be part of core for better usability.

Is there a reason why this should be core and not an add-on in the asset library?

This can likely be implemented with an add-on, but it's not particularly convenient to have to install an add-on to do this.