LibWeb: Introduce color-function-specific style values

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. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -106,11 +106,16 @@ class CSSColorValue;
 class CSSConditionRule;
 class CSSFontFaceRule;
 class CSSGroupingRule;
+class CSSHSL;
+class CSSHWB;
 class CSSImportRule;
 class CSSKeyframeRule;
 class CSSKeyframesRule;
 class CSSKeywordValue;
 class CSSMediaRule;
+class CSSOKLab;
+class CSSOKLCH;
+class CSSRGB;
 class CSSRule;
 class CSSRuleList;
 class CSSStyleDeclaration;