Support at least var(--foo, #rrggbb) fallbacks

Issue Summary

Embedded SVG from Nabla-Regular.ttf does not render. All opaque black. Skia-based renderer (experimental svg hooks to ft2-demos) works just fine.

Example SVG

Will post later. Within Nabla-Regular.ttf from https://fonts.google.com/

Librsvg Version

First noticed on 2.56.2, verified problem with git head (2.56.90-12-g1b589574) too.

Platform

Fedora Linux 38, x86_64

Additional logs

As I mentioned, Skia m110's SVG renderer can handle this. Found this problem while running the two in parallel.

Inkscape also shows opaque black layers.