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.

Simple sRGB color blender test can be used to determine if the sRGB processing
works as expected. It blends alpha ramps of purple and green together such that
at midpoint of image, 50 % blend of both is realized. At that point, sRGB-aware
processing yields a result close to #bbb rather than #888, which is the linear
light blending result.

The demo also contains the sample computation for sRGB premultiplied alpha.