shade/alpha/mix() take colour(s) and a number that is the ratio by which
to transform them. It was written here that these shall be passed in the
order (number, colour). That was wrong: they must be passed in the order
(colour[s], number) to work, and for the Inspector not to flag an error.
This lets us do fallback in case an image format is not
supported, and also lets us provide solid-color images.
We don't support image fragment notations.
See ttps://www.w3.org/TR/css3-images/#image-notation
https://bugzilla.gnome.org/show_bug.cgi?id=761318
