Remove node.matrix

[mre4ce]

A node can currently store a separate translation, rotation and scale or it can store a matrix. I vote for removing the matrix because it forces an implementation to implement code to decompose the matrix into a separate translation, rotation and scale in case an animation only changes some but not all three. Afaik this is the only reason to even have code to decompose a matrix. If for some reason source data stores matrices then it'd be better to do the decomposition in the converter as opposed to adding complexity to the glTF format.

[lexaknyazev]

Can node.matrix contain something besides TRS (such as skew)?

[pjcozzi]

Engines should not need to decompose the matrix for animation; the spec requires that when a node is targeted for animation that it uses TRS since animations can only target those properties.

@mre4ce would you be OK with just a spec update here that make this more explicit?

[mre4ce]

Making this more explicit in the spec would be good thanks. I would still prefer to get rid of node.matrix just to reduce complexity but keeping it is not the end of the world :)

[pjcozzi]

I added a spec clarification in #750.

[donmccurdy]

Addressed in glTF 2.0.