optimize matrix animation

It is common that not all TRS components of a matrix decomposition change during an animation. For example, RiggedSimple has one where only rotation changes and one where nothing changes ( https://github.com/KhronosGroup/glTF-Sample-Models/pull/205 ). It may be possible to skip these no-op animations in the generated glTF for cleaner and smaller files.

Begin by reviewing the converter's glTF matrix-animation generation and the RiggedSimple example in PR #205. Determine how unchanged TRS components and fully no-op animations are represented; done means generated glTF omits eligible no-op animation data while preserving animations with changing rotation or other components.

Written by the indexing model from the issue text.