Draft: Add a COLRv1 glyph renderer

This adds initial support for rendering color glyphs from a COLRv1 table.

The branch is based on the Adrian's svg glyph renderer branch.

For background on COLRv1, see https://learn.microsoft.com/en-us/typography/opentype/spec/colr

This is just a draft, it misses tests, and has only been superficially tests with the test fonts found here: https://github.com/googlefonts/color-fonts

All the freetype API used here is still marked as experimental.