🍒 kcm/output_model: basic mode matching in mirroring
Provide basic support for adapting display modes for output replication.
Suppose the user wants screen A to replicate screen B. We only change the configuration of screen A:
- try to find a candidate mode among all modes with the same aspect ratio. Among those, use the smallest mode at least at big as that of screen B.
- Reposition and rescale screen A to fit in the center of screen B.
This always works when screen A has the same mode as the current mode of screen B. On wayland, this basically works when screen A has modes with matching aspect ratio.
Since X11 does not support per-screen scaling factors, this works poorly.
(cherry picked from commit 7a18812b)