Skins data validation

[lexaknyazev]

From the offline discussion, all skinning-related checks:

Scene Description Issues

• Joints must have one or more common roots.
• skeleton property, if used, must point to a common root.
• When a node with a skinned mesh belongs to a scene, the joint tree must be available in the same scene.
• Local and parent transforms on a node with a skinned mesh are ignored.

Data Issues:

• IBM matrices don't have to be TRS-decomposable.
• Joint indices must be valid (zeros allowed for zero-weight influences).
• Linear sum of weights per vertex is 1.0 (across all influences).

[lexaknyazev]

Implement KhronosGroup/glTF#1352.

[donmccurdy]

Order of weights when an asset uses more than 4 influences.

From latest discussions, the validator probably does not need to verify order of weights, but should raise a warning or hint when more than 4 influences are used.