gltfpack: Validate that accessors have buffer data
gltfpack: Validate that accessors have buffer data

Before this change, accessors could reference buffers with no data, and
the file would have parsed fine but then we'd crash when trying to read
accessor data.

We now validate that all accessors have associated non-dummy buffers,
and additionally correctly validate buffer contents by running
cgltf_validate *after* cgltf_load_buffers.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer