Fixes for output and buffer transforms
Some fixes for issues found while working on direct scanout with pre-rotated buffers:
- crashes for flipped outputs, introduced in !2379 (merged)
- wrongly rotated and positioned hardware cursors on flipped outputs
- broken rendering on flipped outputs
- wrong interpretation of 90/270 buffer transforms (again, see !1055 (merged) for previous attempt)
See rmader/gnome-control-center@a8c214dd for convenient testing of flipped output rotations. Test clients: weston-simpe-egl
/ weston-simple-damage