Enable a user to test and debug multi output configurations on Wayland
without having the available hardware by enabling some basic
configuration of the dummy monitor manager.
Currently available configuration options are:
MUTTER_DEBUG_NUM_DUMMY_MONITORS - to set the number of monitors
MUTTER_DEBUG_DUMMY_MONITOR_SCALES - to configure the monitor scales
See src/backends/meta-monitor-manager-dummy.c for detailed description
of the available configuration parameters.
https://bugzilla.gnome.org/show_bug.cgi?id=747089