x11: keep track of the screen pixel size by calculating the bounding box of monitors

x11: keep track of the screen pixel size by calculating the bounding box of monitors

This is so we always have the latest information given by XRandR (or other), and not
rely on Core protocol information that might not have been updated yet.

This is specially visible when a monitor is connected (less frequent) or disconnected
(much more frequent), callbacks on GdkScreen::monitors-changed that call
gdk_screen_get_width/height() could get the screen size previous to the monitor
rearrangement.

So in order to fix this, keep track of the latest monitors information, and calculate
the bounding box in order to know the screen size.

https://bugzilla.gnome.org/show_bug.cgi?id=715029