gltfpack: Add one at a time exponent encoding
gltfpack: Add one at a time exponent encoding

This may be a better idea for this mode in general since it doesn't make
one component suffer for the sake of others...

For now we always use exp encoding when using -vpf and compression, so:

- vpf + c: one at a time exponent encoding
- vpf + cc: shared exponent encoding
- vpf + no compression: float quantization

Time will tell if this is a good idea.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer