gltfpack: Implement support for point meshes
gltfpack: Implement support for point meshes

We now support point lists as a primitive type for meshes. For now the
point data set is not optimized thoroughly - only quantization is
applied to vertex data.

While in theory it's not very hard to support indexed point meshes,
available test files don't have indexing and there are some minor
complications wrt mesh merging and (future) point reordering.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer