Previously this was implemented inline within the parsing of
`{repeating}-radial-gradient()` functions but it will also be useful for
`circle()` and `ellipse()`.
We now support the CSS Images Module Level 4 additions to the
`<radial-size>` syntax, namely:
- `<length-percentage>` rather than just `<length>` for circles.
- Distinct `<radial-extent>` values for horizontal and vertical for
ellipses.
- Mixing of `<radial-extent>` and `<length-percentage>` values for
ellipses.
The regressions are due to WPT not being updated to expect the first of
these additions.
A few things fall out of this:
- We no longer need to templatize our color-stop list types.
- A bit more code is required to resolve gradient data.
This results in a slightly different rendering for a couple of the test
gradients, with a larger difference between macOS and Linux. I've
expanded the fuzziness factor to cover for it.
We implemented repeating linear gradients by expanding a vector of color
stops until the entire range was covered. This is both a bit wasteful
and caused Skia to draw corrupted gradients to screen whenever the total
amount of color stops and positions exceeded 127.
Instead of doing that, use the original color stops for the shader and
repeat it instead of clamping it. We need to do a bit of math to project
positions correctly, but after that the shader repeats itself nicely.
While we're here, calculate the gradient's length and the center point
as floats instead of ints, yielding a slight but noticeable improvement
in gradient rendering (see the diff on the zig zag pattern in
css-gradients.html for an example of this).
