Couple fixes found by reading the spec:
- Repeating should also happen in negative direction, so the whole
[0, 1] is covered.
- Leftmost and rightmost stops should be clamped to [0, 1] range if
needed, because Skia ignores everything outside of this range.
Skia does not have built-in support for gradient transition hints. So
instead of adding custom gradient painting, now we do the same thing as
other engines and preprocess color stops by replacing transition hints
with a bunch of points lying between adjacent color stops and calculated
using non-linear formula from the spec. As a result we get visually
close enough rendering we would get by applying spec-formula
individually to each point of a gradient.
Skia now uses GPU-accelerated painting on Linux if Vulkan is available.
Most of the performance gain is currently negated by reading the GPU
backend back into RAM to pass it to the Browser process. In the future,
this could be improved by sharing GPU-allocated memory across the
Browser and WebContent processes.
From SkPaint.h:
"Sets the thickness of the pen used by the paint to outline the shape.
A stroke-width of zero is treated as "hairline" width. Hairlines are
always exactly one pixel wide in device space (their thickness does not
change as the canvas is scaled)."
While we expect stroke-width=0 to simply not be painted.
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.
