In a similar way to Position, now specified and computed gradients share
a common Gradient type defined in style::values::generics::image.
This allows us to reuse most code for many style traits like ToCss,
HasViewportPercentage and ToComputedValue.
The test changes are the fallout of the disappearance of AngleOrCorner::None,
which align our code to the spec for serialisation, where components that can
be omitted should be omitted.
To simplify adding additional data to `ParserContext`, this moves test usages to
a few shared locations, instead of being spread across many tests.
MozReview-Commit-ID: 1OahV797eq
Preserve more information from transform function parsing.
Preserve angle unit while parsing.
Simplify SpecifiedMatrix.
Use the write! macro for formatting with a helper called Css.
Implement ToCss for &T if T implements ToCss.
Add some tests and update others.
closes#15194
