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.
* Change pgpu-render header file generator to add a comment noting that the file is auto-generated
* Remove bin target and associated dependencies from piet-scene crate
* Remove FP constructors from the Color type
* Better codegen and rounding in Color::to_premul_u32()
* Add kurbo attribution for piet_scene::geometry module
