Since we scale surface actors given what main output their toplevel
window is on, also scale the window geometry coordinates and sizes
(window->rect size and window->custom_frame_extents.top/left) in order
to make the window geometry represent what is being rendered on the
stage.
https://bugzilla.gnome.org/show_bug.cgi?id=744934
Tracking back from the monitor to the output every time we need to
figure out the scale of a window on a monitor is inconvenient, so
propagate the scale from the output to the monitor it is associated
with.
https://bugzilla.gnome.org/show_bug.cgi?id=744934
