gltfpack: Merge meshes into a multi-primitive mesh
gltfpack: Merge meshes into a multi-primitive mesh

We now merge a consecutive run of meshes that have the same skin/node
assignment into a single mesh with multiple primitives.

This makes sure that we don't generate redundant nodes, and fixes some
corner cases with mesh weight animation where previously a single node
could be split into multiple nodes but animations weren't duplicated to
match.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer