gltfpack: Implement animation resampling
gltfpack: Implement animation resampling

Since we resample all tracks on the same timeline, we only need a single
input accessor which dramatically reduces the amount of time data we
need.

The default animation sampling frequency is 15 Hz which seems like a
reasonable tradeoff between quality and bandwidth.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer