The ChunkIterator now limits a chunk to using only one font (before, it
was possible to have a chunk with >1 font, when `unicode-range` CSS
property is used).
This change allows us to reduce some complexity in the text shaping and
painting code and makes us compatible with the APIs in Skia and
HarfBuzz.
If Metal context and IOSurface are available, Skia painter will use
Ganesh GPU backend on macOS, which is noticeably faster than the default
CPU backend.
Painting pipeline:
1. (WebContent) Allocate IOSurface for backing store
2. (WebContent) Allocate MTLTexture that wraps IOSurface
3. (WebContent) Paint into MTLTexture using Skia
4. (Browser) Wrap IOSurface into Gfx::Painter and use
QPainter/CoreGraphics to blit backing store into viewport.
Things we should improve in the future:
1. Upload textures for images in advance instead of doing that before
every repaint.
2. Teach AppKit client to read directly from IOSurface instead of
copying.
