gtk4: Fix transparency on the GL path
Fixes RGBA for the GL path. GDK texture default is premultiplied alpha, but GStreamer produces non-premultiplied textures, which was causing the artifact.
Unfortunately, as far as I can see, the only way to set a texture format is with a GTK 4.12 texture builder API? And I don't think there's a way to cfg() on a dependency feature without adding another feature. So this is rather ugly. Not sure how to do it better.