This fixes a regression where the geometry label isn't updating even
though the window geometry had changed because the geometry label's
location isn't changing.
An Overlay is similar to a transparent window, but has less overhead
and does not get rendered within the window stack. Basically, the area
that an Overlay occupies forces transparency rendering for any window
underneath, which allows us to render them flicker-free.
This also adds a new API that allows displaying the screen numbers,
e.g. while the user configures the screen layout in DisplaySettings
Because other things like drag&drop or the window-size label are not
yet converted to use this new mechanism, they will be drawn over the
screen-number currently.