Support OT-SVG with CSS var(--color0, ...) and color palettes from CPAL table

In addition to the inlined SVG colors (constant names “yellow” or hex #ABCDEF) which we do support, and the special currentColor keyword, which we don’t yet (cf. #405), OT-SVG table also allows to define color variables (via CSS var() function) that reference color palettes defined in the CPAL table, thus allowing to change the colors by selecting among multiple color palettes (e.g. light vs dark, etc.).

We need to support this both ways: 1) in maximum_color when converting OT-SVG => COLR, and 2) when converting a COLR font with multiple CPAL palettes to OT-SVG (#421).

[litherum]

Please try the Ventura / iOS 16 betas.

[rsheeter]

This is Nabla OT-SVG font (converted from COLRv1 using our maximum_color tool) inside Safari 16.1 on macOS Ventura showing off its alternate CPAL palettes:

AWESOME! Thanks again @litherum 😃 I will consider myself in your debt by a beer [or your preferred beverage] or two 😄