Implement parsing support for KHR_meshopt_compression

[zeux]

Specification: https://github.com/KhronosGroup/glTF/pull/2517

Because the JSON structure matches that of EXT_meshopt_compression, and both structures can't be present on the same buffer view, we reuse the existing C structure and merely tag the object "is_khr", which could be useful to decoding -- although the updated vertex data will be automatically decompressed with a meshoptimized update without code changes.

In addition to vertex encoding v1, which will be decoded automatically, KHR_meshopt_compression adds support for color filters which requires an extra enum entry.