This makes show_image() deal with more formats than just a8r8g8b8, in
particular, a8r8g8b8_sRGB can now be handled.

Images that are passed to show_image with a format of a8r8g8b8_sRGB
are displayed without modification under the assumption that the
monitor is approximately sRGB.

Images with a format of a8r8g8b8 are also displayed without
modification since many other users of show_image() have been
generating essentially sRGB data with this format. Other formats are
also assumed to be gamma compressed; these are converted to a8r8g8b8
before being displayed.

With these changes, srgb-test.c doesn't need to do its own conversion
anymore.