Hmm. Telling xvfb to default to a TrueColor visual did help, in that
it got rid of the bogus backgrounds on all the text; but on the
other hand it mysteriously caused all the images to become black and
white! Serves me right for testing with Bridges which was B&W to
start with. Instead, we'll just tell xvfb to use a 24-bit display
and let it sort out the visuals for itself; that seems to work better.

[originally from svn r7932]