Try to tell Macs with two graphics cards that it's ok to
use the low-performance one for the GC bitmap (when
GL is used for that, which is OS X 10.11 and later).
The GC blit implementation used on Mc OS X 10.11 assumed that
the no-GC bitmap is blank. Make it use the given no-GC bitmap.
Also, repair the left-to-right flipping(!) of the GC bitmap,
and repair a backing-scale mismatch that could leave a thin
border around a GC blit.
Previous versions of OS X seem to have automatically disabled
update for a window when flushing is disabled, but now we have to
do it ourselves, otherwise text editors can flicker a lot. (The
system change is probably related to the removal of the
`isFlushWindowDisabled` method of `NSWindow` (which was
deprecated in 10.10).
The old strategy of switching a transparent window to solid and back
doesn't work on 10.11; it appears that queued messages must be handled
for the window to become visible, but that's not allowed during a GC.
The strategy for 10.11 and up create an OpenGL canvas, which acts as a
direct-to-screen drawing area that a GC callback can affect without
Racket-level allocation.
