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.
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.
