gltfpack: Use more rigorous bone influence filter
gltfpack: Use more rigorous bone influence filter

Instead of processing bone influences separately for each group (which
only works if we have one group), we now run a separate filter pass
that, in addition to sorting by bone index (for compression), sorts by
bone weight first and discards all but 4 dominating weights.

Additionally we renormalize the 4 weights before quantizing and
renormalizing in unorm8 space. This is helpful to deal with models that
have *really* incorrect weights that add up to, say, 0.5 - by
normalizing in floating point first, our unorm8 normalizer becomes more
precise.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer