[Xcb] Keithp fixes util/image byteswapping

[Barton C Massey]

Huge thanks to Keithp, who just did some very substantial
changes to util/image to make it verifiably work in
big-endian and mixed-endian settings.  The verification is
provided by a new and very thorough test he wrote.  He also
cleaned up a ton of cruft I'd put in there in the process.

Thank goodness.  I don't think I would have ever figured it
out.

Ulrich, your bug should be closed now. :-)

    Bart