When the last output gets disconnected, create a virtual output as a
placeholder until we have access to a physical output again. While
this placecholder never gets rendered to, with virtual outputs in
general that is possible (with gbm and qpainter atm) and can be done
for future use cases like wireless displays.
CCBUG: 420160
CCBUG: 438839
DrmPipeline is now what contains all the thing related to drm calls,
instead of DrmOutput. This allows for some more flexibility and tidies
the code up a bit. Additionally instead of rolling back changes if
presentation fails, changes are directly tested with atomic test only
commits.
This reverts commit 5a22deda3b.
We still need more work to finish the DrmPipeline. At the moment, there
are a few major issues, e.g. some outputs not turning on, output
transforms not working correctly, a crash when changing dpms mode.
Let's merge this change back once all major issues are fixed and after
more testing.
DrmPipeline is what now contains all the drm bits related to
modesetting and presentation, instead of that being in DrmOutput.
This gives a lot more freedom for managing drm resources and
enables far better usage of the atomic API with guaranteed
immutability for failed tests.
