gtk4: Support RGBx formats in SW paths
GTK4 has matching enums and thus should handle them fine. Further more it should allow renderers to reduce memory bandwidth by applying occlusion culling.
See also glcolorconvert: Prefer xRGB formats if downstre... (gstreamer#3279) and https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/7425
Edited by Robert Mader