The new system control font is (intentially) not associated with a
font family, so it's inaccessible via Pango. A patch to Pango lets
us synthesize and register new families, so the control font can
be made accessible.
Closes PR 15121
A better solution would be to implement the dialog, but given
how long it's been broken, maybe no one cares enough for that
to be worthwhile.
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.
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
