Adds kurbo as an optional dependency and implements conversions to/from the common types.
This also removes the direct pinot dependency and changes moscato (temporarily) to a git based dep to allow iteration on the underlying glyph loading code without PR churn here.
Adds a test to visualize the blend modes. Fixes a dumb bug in blend.h and also a more subtle issue where default blending is not the same as clipping, as the former needs to always push a blend group (to cause isolation) and the latter does not. This might be something we need to get back to.
This should fix the rendering, so it fairly closely resembles the Mozilla reference image. There's also a compile-time switch to disable sRGB conversion, which is (sadly) needed for compatible rendering.
