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.
The Web::CSS::Parser's GradientParsing ignores color-stops if
it is only a single one. This change allows to have color-stops
with double positions against a single color.
Further, also allows for `linear-gradient(black)` and similar
other gradient functions
