Previously, each GHOST Context instantiated its own Metal device
queue. Commands are only synchronized within a queue, this was the
root cause of a number of flickering issues which had previously
been worked-around with synchronization primitives.
New solution uses a shared queue to simplify dependencies and
alleviate possibility of stalls and bugs when resources are modified
or shared across separate GPU command queues.
Authored by Apple: Michael Parkin-White
Pull Request: https://projects.blender.org/blender/blender/pulls/108223
9e83211b8d