Virtualised outputs use the "suggested X" and "suggested Y" DRM props to
indicate their relative position on the host WM.
This change adds members `suggested_x` and `suggested_y` to `struct
wlr_output` which contain the value of these props. Also, a
`suggested_position` signal is added so that compositors can be notified
if there is a changed in the suggested position.
Legacy gamma lut size now uses the new legacy_crtc member of
wlr_drm_crtc. This was Previously doen using old_crtc in
wlr_drm_connector, but since this refers to the crtc that was connected to
the ouput, this could give the wrong result.
