For now, only impls for types like in style::values::generics can be derived.
This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.
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.
A specified position is now a struct made of two values of different types,
the first one being PositionComponent<X>, and the second one PositionComponent<Y>.
A position component is represented by the new enum PositionComponent<Side>,
with the three values Center, Length(LengthOrPercentage), and
Side(Side, Option<LengthOrPercentage>).
Side keywords are represented by the X and Y enums, which don't include a value
for the center keyword anymore. They are accompanied by the Side trait, which
allows us to determine whether a side keyword is "left" or "top".
This refactor simplified the parsing and serialisation code and exposed bugs in it,
where it would reject valid <position> values followed by arbitrary tokens,
and where it would fail to prefer "left" to "right" when serialising positions
in basic shapes.
