These cannot be implemented with the standard OpenGL blending functions, so we
have to do them manually in a shader, which requires a good deal of machinery
to create intermediate framebuffers and so forth.
Also, this commit rewrites the `add_fill()` method to stop using shuffle
instructions, which can be slow and make the code overly complicated. The
shuffle instructions have been removed from the various SIMD backends.
This commit substantially reworks the `pathfinder_gpu` API to better support
modern APIs like Metal. It should open the door to `gfx-rs`, `wgpu`, Vulkan,
and D3D12 backends relatively straightforwardly.
A new example, `canvas_metal_minimal`, has been added.
Note that the new Metal shaders require a patched version of `spirv-cross` to
build properly. An upstream patch is forthcoming.