We can do more tile-based optimization, but this should be enough to ensure a
correct rendering.
This temporarily breaks subpixel AA, but it should be fixable by modifying
`blur.fs.glsl`.
Closes#271.
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.