reftest-compare: Treat colour channels as undefined if alpha is zero

reftest-compare: Treat colour channels as undefined if alpha is zero

If the alpha channel is zero, it doesn't matter what the values of the
red, green and blue channels are: the pixel is still fully transparent.
On most architectures, fully transparent pixels end up all-zeroes
(fully transparent black), matching what's in the reference PNG file;
but on mips*el the blend-difference and blend-normal tests get all-ones
(fully transparent white) and a test failure.

Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/4227
Signed-off-by: Simon McVittie <smcv@debian.org>