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: #4227 (closed)