Blender Git Loki

Attempt to fix #35057, disable threading if diameter of the brush
becomes too small. Typically this would happen if the number of buckets
is clipped to the maximum value. This avoids thread overhead.

A better fix might be to do bucket-brush intersection on main thread and
dispatch threads to process bucket hits as they become available. This
way only one thread at most would end up being used in such cases
anyway. A better task scheduler is needed for that though, leaving for
after GSOC.