optimize matrix animation

[andreasplesch]

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 ( KhronosGroup/glTF-Sample-Models#205 ). It may be possible to skip these no-op animations in the generated glTF for cleaner and smaller files.

[lasalvavida]

Thanks for the suggestion @andreasplesch, I agree that this would be good to have.