Support CSS var() for custom colors and other SVG properties

Somewhat related to #379 (closed), it would be useful to support CSS variables, the var(--variable-name, fallback-value) syntax for property values.

This needs either some extra API to provide values for the variables, or maybe reuse the API from #379 (closed) if one assumes that the custom CSS provided by the caller has something like this:

:root {
  --variable-name: value;
  --another-variable-name: another-value;
}

SVG Native (a reduced version of SVG used for OpenType emoji and such) will be using var() to provide caller-controled coloring of SVG elements. GNOME/GTK may be able to use the same technique.

CC @jimmac for feedback - how would this work for you?