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.

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See also glcolorconvert: Prefer xRGB formats if downstre... (gstreamer#3279) and https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/7425

cc @slomo @company