OMI_physics_joint

[aaronfranke]

Preview: https://github.com/aaronfranke/gltf-extensions/tree/OMI_physics_joint/extensions/2.0/OMI_physics_joint

This PR adds a new spec for physics joints.

Overall I like Microsoft's spec. This spec is similar with a few differences:

• Joints are a separate spec instead of part of the rigid body spec.
• Joints are their own nodes. This allows the joint's position to be defined by the transform of the glTF node it's on, which is much more intuitive. This is how Godot, Unreal, Roblox, and Blender work, but Unity has joints as part of a RigidBody node. It looks like Unity is the odd one out here, and Unity's way is less intuitive.
• Some renames, min -> lowerLimit, max -> upperLimit, springConstant -> stiffness, springDamping -> damping.
• It would also be good to continue discussing this list Questions about joints eoineoineoin/glTF_Physics#15

This PR includes a detailed spec, several example files, and a Godot Engine implementation with full support for import and export. The Godot implementation is in this ZIP: OMI_physics_joint.zip and here omigroup/omi-godot#1

Validator: omigroup/glTF-Validator#3

[robertlong]

The vote passes to move to stage 1