Based on first patches by Christian Hergert.  Change
screen_get_monitor_geometry() so that it translates the layout of the screens
from Cocoa layout to GDK layout.  In Cocoa, the screen locations
are specified in Cocoa geometry, as well as that GDK uses a different way
to place individual monitors in the root window.  For now only monitors
that are laid out horizontally are supported (see the FIXMEs in the source),
in bug 596238 we will track future work to get things fully right.

Modify _gdk_quartz_window_get_inverted_screen_y() to take the differences
in screen layout between Cocoa and GDK into account.  Also this function
is subject to future work.