MonitorManager: further extend the dummy backend

The default configuration is extended, which is only possible
if there are as many CRTCs as outputs, so make sure that's true.

Also, add more and bigger modes, so that different sizes will
be chosen for the three outputs.
A nice side effect of this is that with a real 1920x1080 + 1600x900
layout, if you disable the VGA you get a stage that matches the
screen size, which triggers the legacy fullscreen path in the
outside mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=705670