Nicer colors for border-style: ridge/groove/inset/outset

[SimonSapin]

These border styles need to generate (at least) two shading colors from only one specified border color. WeasyPrint just add/substracts 50% to each RGB channel, but the results are not great.

Here is on Firefox does it. I’m not sure why it’s also based on the background color.
http://mxr.mozilla.org/mozilla-central/source/layout/base/nsCSSColorUtils.cpp#40

[liZe]

I'm quite happy with this new algorithm based on HSV colors. It works well with black, white, gray, saturated and dark colors, but light colors are not darkened very well (nothing's perfect).