Blender Git Loki

Volumes: fix calling BKE_volume_load from multiple threads

`BKE_volume_is_loaded` uses `grids.filepath` to determine if the
grids are already loaded. The issue was that `grids.filepath` was
set before the grids were loaded, resulting in incorrect early
returns for other threads.

Differential Revision: https://developer.blender.org/D10150