Before this change we were serializing them in a bogus 8-digit hex color
format that isn't actually recognized by HTML.
This code will need more work when we start supporting color spaces
other than sRGB.
Instead of CSSColorValue holding a Gfx::Color, make it an abstract class
with subclasses for each different color function, to match the Typed-OM
spec. This means moving the color calculations from the parsing code to
the `to_color()` method on the style value.
This lets us have calc() inside a color function, instead of having to
fully resolve the color at parse time. The canvas fillStyle tests have
been updated to reflect this.
The other test change is Screenshot/css-color-functions.html: previously
we produced slightly different colors for an alpha of 0.5 and one of
50%, and this incorrect behavior was baked into the test. So now it's
more correct. :^)
Checking that the string parsed for the `font` property is not enough,
the spec also wants to rule out CSS-wide keywords like `inherit`. The
simplest way to do so is to check if it's a ShorthandStyleValue, which
also rules out use of `var()`; this matches other browsers' behaviour.
The newly-added test would previously crash, and now doesn't. :^)
