Refactor chunk mesh updating code

[kpreid]

Clean up the relationship of the chunk_updater() function in ChunkedSpaceMesh and ChunkMesh::recompute(); they should probably become just one function, or otherwise become something quite different.

https://github.com/kpreid/all-is-cubes/blob/27c96dce860d496393fa52ca999e111a2b3ad8ea/all-is-cubes-mesh/src/dynamic/chunked_mesh.rs#L453-L457