Output: make sure to have a currentMode
!5761 (diffs) seems to be the origin for https://crash-reports.kde.org/organizations/kde/issues/53606/?project=12&query=is%3Aunresolved+issue.priority%3A%5Bhigh%2C+medium%5D&referrer=issue-stream&statsPeriod=30d&stream_index=1
Previously currentMode was always part of the available modes.
With the desiredRefreshRate/desiredSize, the current mode might not be a known mode.
The commit makes sure to transmit the currentMode and that it added to the Output modes.
I haven't validated this fixes the crash.