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.
This lets us not care about non-absolute Length units when resolving
gradient data, as they'll already have been converted to px.
We can also use Angle::from_style_value() safely on absolutized angles,
which reduces some boilerplate code.
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.
When setting style to a CSSStyleValue we need to convert it to a
StyleValue. If we already have one, we might as well use it avoid the
work of serialization and re-parsing.
I realised I misunderstood what "constructed from a USVString" means, so
I've adjusted based on that. It does raise a question on what the source
USVString is if that string resulted in multiple CSSStyleValues being
created - see the linked issue.
