WIP: BE format fixes

Apparently we end up with a quite bonkers channel order inside generate_unswizzled_blend. Mostly the alpha channel is at the wrong position leading to quite wrong colors in glxgears and compositors when eg starting the X server with a color depth of 16.

Not quite sure where the actual issue lies and it might be related to the big endian encoding we have in our u_format.csv file.

Anyway, in case somebody has a good idea on what's up here so we can fix this issue more generally as other packed formats are also quite broken, but with different issues.