onscreen/native: Hold output and CRTC refs until dispose
We relied on them being valid longer to keep track of used GPUs. If we don't have the CRTC (or output) we don't have a way to fetch the pointer to the MetaGpu that drives the associated monitor.
This avoids a crash when trying to fetch said pointer from what would be the NULL MetaCrtc pointer.
Fixes: 08593ea8 ("onscreen/native: Hold ref to the output and CRTC until detached") Closes: #2667 (closed)