scene-helpers: use pending_commit_damage, chases wlr!4253

scene-helpers: use pending_commit_damage, chases wlr!4253

Using the output damage_ring for early out will break VRR in
direct scanout mode. The reason being that the damage_ring
will be completely ignored in that mode so we need to check
`output->pending_commit_damage` instead. This matches with
what wlroots has been doing since [0] and it was missed in
the initial port to wlroots 0.18.x.

However, that would then break the magnifier which only adds
its damage to the damage ring. After some discussion with
the wlroots devs we came up with a solution that should work
for both, wlroots 0.18.0 and when [1] is backported to 0.18.1.

Note that even with this PR, VRR in direct scanout mode is
broken in 0.18.0 from the wlroots side and will be fixed once
[1] is backported to the 0.18 branch and 0.18.1 is released.

Fixes: #2078
[0] https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4253
[1] https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4753