Use std::move in MeshAssimp.
Use std::move in MeshAssimp.

Tested this via material_sandbox, seems to be fine.

Fixes #1366.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament