Reported by @fclem. On Metal GPU_finish enacts a CPU<-> GPU sync by
submitting a command buffer and waiting on the completion event.
However if there was no command buffer to submit then the call was
returning immediately, regardless of any outstanding GPU work.
This fixes that case by keeping track of all outstanding work and
blocking on that.
Authored by Apple: James McCarthy
Co-authored-by: James McCarthy <jamesmccarthy@apple.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/128987
6da42e9c95