cogl: Revert swizzling for BGRA buffers
As it was originally reported on https://bugzilla.gnome.org/show_bug.cgi?id=779234#c0, the hottest path was convert_ubyte() in mesa. Reverting this shows no trace of those hot paths, nor any higher than usual CPU activity.
As the improvements at the time were real, I can only conclude that pixel conversion was happening somewhere further the pipeline, and swizzling just helped indirectly. That got eventually fixed, so swizzling just stayed to cause grief. And lots it caused.
Time to bin this, it seems.