Convert -webkit-linear-gradient coordinate system for Stylo
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This patch will fix the -webkit-linear-gradient direction issue in Stylo. I decided to NOT change the behavior of Servo, because according to the webcompat spec the webkit prefixed version should just be an alias of non-prefixed version. But we might want to remove support for old syntax like "-webkit-linear-gradient(top, ...)" (without "to"), because they are not in the spec anymore.
The Gecko layout system use different coordinate systems for prefixed and non-prefixed version, so I did a conversion, but for Servo I keep the modern version only.
Since I believe we should not support old syntax in Servo, so I'm not going to put the test in Servo. I'll submit my test cases to the Gecko wpt test instead.
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- [x] `./mach build -d` does not report any errors
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- [x] These changes fix [Bug 1372821](https://bugzilla.mozilla.org/show_bug.cgi?id=1372821)
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because test will be submitted to the Gecko side
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We simply implement ToCss for Option<T>, printing nothing if the value is None,
and we then use SequenceWriter to skip writing of separators around empty parts.
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.
