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.
The Metal shaders are checked into the repository for convenience, but they
should not be directly edited. Instead, edit the corresponding GLSL shader in
the top-level `shaders/` directory, and rerun `make` in there.
Patrick Walton