The code parsing interpolation methods hadn't learned about
or latest color space additions. While we're at it, improve
the error reporting a bit.
Tests included.
Currently we only have sRGB, so it's a bit redundant, but we'll need this
for color-mix()
Once we have more color spaces, they should be added here (presumably the
enum would be in GDK instead, and instead of GdkRGBA these colors would
have a GdkColor.
Make calc() work in colors too, since we need to support degrees for hsl()
hue anyway and it goes through the same machinery. Make that work for
legacy syntax too, matching the spec.
Ignore missing components/none for now.
Ignore gdk_rgba_parser_parse(), that's also used outside css.
