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.
Closes: https://gitlab.gnome.org/GNOME/gtk/issues/1737
Don't export any functions taking or returning MacOS types in
gdkquartz.h, gdkprivate-quartz.h, or any header that either includes.
The GdkQuartz internal functions are moved to a new header
gdkinternal-quartz.h, the functions used by quartz-specific
Gtk files are moved to another new header gdkquartz-gtk-only.h, and
the key and event enums to a new header gdkkeys-quartz.h.
We've long had double precision mouse coordinates on wayland (e.g.
when rotating a window) but with the new scaling we even have it on
X (and, its also in Xinput2), so convert all the internal mouse/device
position getters to use doubles and add new accessors for the
public APIs that take doubles instead of ints.
