Don't try to mirror the physical dimension, since that's a property of
one of the monitors, not of the logical monitor. Callers are changed to
deal with choosing the monitor to represent the logical monitor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
Instead of using crtcs and outputs to generate logical monitors, use
the ready made monitor abstraction that hides irrelevant things such as
monitor tiling etc.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
