[ENH] Support for COLRv1-encoded colour glyphs #31210

These are similar to COLRv0, but instead of simple glyph layers, additional compositing operations and gradients are also available. Unfortunately, this means that FreeType does not directly support this format as it requires a full renderer. FreeType does expose all of this data for us.

We of course do have a full renderer available in Agg, but wiring up all of these operations is likely to be a bit of work.