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/HTML/HTMLMetaElement.cpp

@@ -76,7 +76,7 @@ void HTMLMetaElement::inserted()
         auto css_value = parse_css_value(context, value, CSS::PropertyID::Color);
         if (css_value.is_null() || !css_value->is_color())
             return;
-        auto color = css_value->as_color().color();
+        auto color = css_value->to_color({}); // TODO: Pass a layout node?
 
         // 4. If color is not failure, then return color.
         document().page().client().page_did_change_theme_color(color);