Before this change, calc() would resolve to different types depending on
the nearest containing value context. This meant that rgb(calc(), ...)
by itself worked correctly due to fallbacks, but rgb(calc(), ...) inside
e.g a linear-gradient would create a calc() value that resolves to a
length, which subsequently got rejected by the color value parser.
Fixing this makes various little gradients show up on Discord.
038d8ade50