Even rendering "flat colored shapes" efficiently can be a bit non-trivial if you expect pixel-perfect results, like you'd get by plotting to an ordinary framebuffer - the GPU's fixed rendering pipeline is not generally built for that. The emerging approach is to use compute shaders, and these are not yet fully integrated with existing programming languages - you can't just edit ordinary C++/Rust code and have it seamlessly compile for CPU and GPU rendering. But we're getting closer to that.