Otherwise, we just keep painting into the old backing store. This fixes
an issue where the main spreadsheet area in Google Sheets was not
visually updating, despite everything looking good in memory.
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. :^)
