Fix GCD deadlock on sync queue draw

[jdtsmith]

TLDR: A queue attempting to flush itself from the "inside" with a call to dispatch_sync deadlocks, so don't do that.

Another attempt to fix the dreaded (but rare) GCD draw deadlock, that often (but not always) occurs on waking from sleep. See #78.

Here we assume the problem arises from a serial queue deadlock which occurs when a block from the queue running on the main GUI thread attempts to use dispatch_sync on the same queue.

So an asynchronous block on the global_focus_drawing_queue tries to "flush" the queue, using dispatch_sync to quickly wait for it to complete all its submitted blocks. But if this happens on the main thread, dispatch_sync is waiting for the queue to drain, and the currently executing block on the selfsame queue is waiting for dispatch_sync to complete: deadlock!

Apparently this isn't a modern approach to drain GCD queues, but I kept it intact, and simply do nothing to sync if we are calling in to the same queue.

See also #86 for another (failed) attempt.

• src/macappkit.m (set_global_focus_view_frame): set identifier key on
  drawing queue for easy identification.
  (mac_draw_queue_sync): Do not dispatch empty block if already running
  from drawing queue.

[jdtsmith]

I have had no recurrences of the issue in #78, so I am again cautiously optimistic that this is the right fix.