Testing support for COLR outlines

[dfrg]

Testing is currently non-existent for color outlines. The plan is to build something similar to extract_glyphs.py and use FreeType to generate serialized paint graphs that can be tested against our code.

We'll likely want to start with the test font available at https://github.com/googlefonts/color-fonts/blob/main/fonts/test_glyphs-glyf_colr_1_variable.ttf. The build script for that font has useful info.