chase wlroots: simplify transforms in scene-helpers (MR 4819/4845)

chase wlroots: simplify transforms in scene-helpers (MR 4819/4845)

Ref: 009515161bd97d8f920d72d31ef462f2608688e8
("scene: Only accept buffer coordinates for damage")

Note:
MR 4819 is immediately followed by MR 4845.
MR 4819 removes return value of wlr_damage_ring_add().
MR 4845 applies applies buffer-local coordinates for
scene_output->damage_ring instead of transformed coordinates.