Have you tried splitting the area covered by each shape into several 8x8 pixel rectangles, then running a compute shader over each one that executes the exact s

Have you tried splitting the area covered by each shape into several 8x8 pixel rectangles, then running a compute shader over each one that executes the exact same rasterisation algorithm as Flash did? That's more or less how triangles are rasterised on the GPU anyway. It's definitely not a simple solution, but might enable you to do runtime rasterisation with a good framerate on the GPU rather than pre-rasterising all the vector art.