Explore alternatives to animation baking.

[zellski]

We currently step through each clip's duration at a constant rate, ask the FBX SDK to calculate the transform of every node, and slam any non-trivial values into a buffer. This leads to enormous files for complex figures. There are many cases now where we can probably represent the animations much more like they're structured in the FBX world.

The main complexity (apart from just writing lots of new code) is probably the bewildering array of features FBX offers for cubic interpolation mode, most of them having to do with the behavior of tangents around keyframes. Figuring out which of them can be represented in glTF (and with how much work) might be non-trivial.