Support KHR_meshopt_compression

`KHR_meshopt_compression` is a successor to the existing extension `EXT_meshopt_compression`.

> On typical geometry data, enhanced attribute compression provides approximately 10% geomean reduction in vertex data size; the gains obviously highly depend on the content. On point clouds, as well as some 3D scanned models that use vertex colors, the new color filter together with the new attribute compression results in ~15% geomean reduction, with even stronger (20-25%) gains when non-aligned bit counts are used (e.g. 6-bit or 10-bit colors).

See https://github.com/KhronosGroup/glTF/pull/2517 for more details.