Tracking: Skia backend

[nicoburns]

This issue tracks the state/progress on the Skia.

Rendering features

• Stroke
• Solid fill
• Gradients
• Images
• Glyphs
• Clipping
• Blending
• Blurred rounded rects

Backends

• OpenGL
• Metal
• Vulkan (experimental, needs a polish)
• DirectX

Performance

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

Quality

• Use linear image sampling

Bugs

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