Change the values emitted for incorrect pixels to make small errors visible
auto merge of #1570 : larsbergstrom/servo/reftest_better, r=metajack

Previously, small errors were very hard to see because they would only have a value that was off by a few points from solid white. Now, they will show up vibrantly!

I considered reconstructing the pixels and using either black/white or a scaling "red" but in practice this gives good results on a wide variety of errors and has the bonus that if you've got chroma-power you can tell if there's an R, G, or B error :-)

r? @metajack