monitor-manager/xrandr: Use a single supported scales list for all

monitor-manager/xrandr: Use a single supported scales list for all

Under X11 we can only ever have the same scale configured on all
monitors. In order to use e.g. scale 2 when there is a HiDPI monitor
connected, we must not disallow it because there is a monitor that does
not support scale 2. Thus we must show the same scale for every monitor
and monitor mode, even though it might result in a bad experience.

Do this by iterating through all the monitors adding all supported
scales by the preferred mode, combining all the supported scales. This
supported scales list is then used for all monitor and modes no matter
what.

https://bugzilla.gnome.org/show_bug.cgi?id=788901
https://launchpad.net/bugs/1724024

Gbp-Pq: Name 0014-Use-a-single-supported-scales.patch