More complete drm autotests
The current autotests for the drm backend are relatively limited:
- it only uses QPainter
- it only tests a few drm-specific scenarios
- it can't present multiple consecutive frames because frame callbacks aren't implemented
What we'd like to do are things like
- test color management
- test screen transforms
- test the hardware cursor
- test the software cursor vs the hardware cursor
- test direct scanout, specifically with screen transforms, clipping and scaling
- (eventually) test KMS offloading of color management features
For that we'll want to
- have VKMS in CI
- set up full KWin, using the drm backend
- execute whatever test steps
- read back the result from writeback connectors, and verify against some known or desired state
- make sure that image comparison doesn't have false negatives all the time