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.
If there is any longhand consisting of both keyword and position, we
should attach both keyword and position for both longhands when serialize
them.
1. To generate a serialized result with keyword, we add a macro function
`to_css_with_keyword` accepted HorizontalPosition and VerticalPosition as
the parameter to finish that. For the longhands which missing keyword, we
use the default keyword.
(`left` for `*-position-x` and `top` for `*-position-y`)
2. Update `Position::to_css` to generate the valid format by calling
`to_css_with_keyword` with HorizontalPosition and VerticalPosition.
3. Update `to_css` to use new `Position::to_css` in background,
background-position, mask and mask-position.
MozReview-Commit-ID: 5Bnhdsi5yeM
Invalid three value positions are no longer accepted
<!-- Please describe your changes on the following line: -->
- Position parse no longer accepts invalid three value positions
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- [X] These changes fix#15488 .
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This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.
Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.
Then we should make the font provider mandatory, and implement the missing stylo
bits.
