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] `./mach test-tidy` does not report any errors
- [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.
