- ImBuf reference counting: turn that into just an atomic integer
- Cachefile safety: turn into a mutex, since work under the spinlock
was quite heavy (hashtable creation, other memory allocations)
- Movie clip editor: turn into a mutex, since work under the spinlock
was very heavy (reading files from disk, etc.)
- Mesh intersect: remove the previously commented out spinlock path;
replace BLI mutex with C++ mutex for shorter code
Pull Request: https://projects.blender.org/blender/blender/pulls/137989
