Description of morph targets in mesh.primitive.schema.json is misleading

[UX3D-haertl]

https://github.com/KhronosGroup/glTF/blob/36f32a21242b25545cd7c142cb81be6cbc614233/specification/2.0/schema/mesh.primitive.schema.json#L80

A plain JSON object specifying attributes displacements in a morph target, where each key corresponds to one of the three supported attribute semantic (POSITION, NORMAL, or TANGENT) and each value is the index of the accessor containing the attribute displacements' data.

Morphing texcoord and color attributes is supported by the spec and it is also implemented in sample viewer. Reading the description one could think this is not the case. This was also pointed out by https://github.com/KhronosGroup/glTF/issues/1363#issuecomment-429013378 but was not changed.

The wording is probably the reason why glTF-Validator reports an error for using texcoord or color as morph targets. Related issue: https://github.com/KhronosGroup/glTF-Validator/issues/213