Deterministic chunk mesh ordering

[kpreid]

I added parallel chunk updates without thinking very hard about the consequences, and now the glTF export fails nondeterministically, because ordering in the file is determined by ordering of the parallel operations.

Possible solutions:

• Have the glTF exporter sort the meshes by their MeshIds.
• Give ChunkedSpaceMesh a method to request deterministic updates (which could still be calculated in parallel, but with sorted sequential callbacks).

[kpreid]

Fixed by sorting in 590de4d.