In case of an offscreen window find its onscreen embedder, and NULL
any GdkWindowImpl that's not quartz, checking all return values.
This replaces 16ded683, removing its unnecessary search_for_nearest
functions.
Any unchecked cast of a GdkWindowImpl to GdkWindowImplQuartz risks at
least getting an invalid member reference, not just the ones in
gdk_quartz_window_get_foo, and all calls to gdk_quartz_window_get_foo need
to be checked, not just Gtk's internal calls. The motivation was that
transient_for was getting set on a Gimp offscreen window and that caused a
crash in raise_transient.
Some applications need to access gdk_quartz_window_get_nsview,
gdk_quartz_window_get_nswindow, and gdk_quartz_event_get_nsevent
so move these from the private gdkquartz-gtk-only.h to a new
header gdkquartz-cocoa-access.h. Don't include this header in
gdkquartz.h so that user code that doesn't need to access these
functins isn't required to compile with Objective C/C++.
Closes https://gitlab.gnome.org/GNOME/gtk/-/issues/1737
Current problems:
* other widgets in a GL-painted window are low-resolution on Retina
display
* something wrong with paint updates; gdkgears demo only updates every
couple of seconds but reports ~30fps
See: #517
