Tracking: Skia backend

This issue tracks the state/progress on the Skia.

### Rendering features
- [x] Stroke
- [x] Solid fill
- [x] Gradients
- [x] Images
- [x] Glyphs
- [x] Clipping
- [x] Blending
- [x] Blurred rounded rects

### Backends
- [x] OpenGL
- [x] Metal
- [x] Vulkan (experimental, needs a polish)
- [ ] DirectX

### Performance
- [x] Cache Typefaces
- [x] Use box shadows rather than image filters for shadows
- [x] Typeface instances
- [x] Cache Fonts
- [x] Cache Images
- [x] Reuse Paint instances
- [x] Reuse glyph vecs
- [ ] Optimise clip-only layers
- [ ] Optimise rect paths

### Quality

- [x] Use linear image sampling

### Bugs
- [x] Vulkan corruption
- [x] Variable font axes (temporary hack in place)
- [x] Map from -1 to 1 not 0 to 1
- [x] TTC fonts don't render on macOS