Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
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.
