The `fullscreen` method implied `show` on Mac OS X and Windows, but
not on Gtk, and a frae shown that was was not properly registered
as shown (e.g., the application could exit). For consistency, adjust
`fullscreen` to not imply `show`. Also, shift to a remaining frame
on Mac OS X when a fullscreen frame is closed.
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.
