backend-drm: add fallback-mode for outputs without modes
If a monitor has broken or invalid EDID, Weston will not be able to populate the modes on the output and fail to enable the output.
To handle this situation, a fixed modeline can be set for such outputs using the mode=
option in weston.ini
. However, this modeline will always be used even if a monitor with correct EDID is connected. In this case, a different mode might be preferable over the fixed modeline.
Add a fallback-mode=
option that accepts a modeline that is only used, if Weston failed to populate the modes from the EDID.