Testing support for COLR outlines

Testing is currently non-existent for color outlines. The plan is to build something similar to [extract_glyphs.py](https://github.com/googlefonts/fontations/blob/c84689133f0565d50e02a90e8824450b8aab2a9a/font-test-data/test_data/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](https://github.com/googlefonts/color-fonts/blob/main/config/test_glyphs-glyf_colr_1.py) for that font has useful info.