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.
Patrick Walton