Quoting the main commit:
If no viewport is set, the neutral viewport is the surface size
without viewport destination size applied - i.e. transform and
scale applied to the buffer size. Change it accordingly, giving
us the same values we'd return in `get_width` in this case.
As result, this only changes cases where a viewport destination
size but no viewport source rectangle is set.
The change fixes exactly such cases, e.g. the Gstreamer Wayland
sink. Can be tested with: `gst-play-1.0 --videosink=waylandsink`.Regarding the last commit: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1446 makes the gstreamer waylandsink use buffer damage as well.