When placing a popup and the legacy transient wl_shell_surface surfaces,
take the current scale of the window into account. This commit doesn't
fix relative positioning in case a window scale would change, but since
the use case for relative positioning is mostly popups, which would be
dismissed before the parent window would be moved, it should not be that
much of a problem.
https://bugzilla.gnome.org/show_bug.cgi?id=744934
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
