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).
Call paint_border() recursively, once for the outer line, and once for
the inner one. This is done in a lambda so that we can reuse it for a
couple of other line styles.
Border-radius behaviour doesn't match other browsers, and goes a bit
haywire in some cases. I've left some FIXMEs for someone who
understands the maths here better than I do. 😅
The LineStyle handling is moved to the start of the function, to avoid
unnecessary work.
With the newly supported fuzzy matching in our test-web runner, we can
now define the expected maximum color channel and pixel count errors per
failing test and set a baseline they should not exceed.
The figures I added to these tests all come from my macOS M4 machine.
Most discrepancies seem to come from color calculations being slightly
off.
