Fix linear gradient's specified form #13892
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WIP for #13892
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Stylo: Basic support for grid-{row,column}-{start,end}
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Implement background-position-x/y
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This is a WIP PR. Just HorizontalPosition / VerticalPosition implementations are complete. I would like to get early feedbacks about this architecture. Here's some architectural topics to consider:
- I created `HorizontalPosition` and `VerticalPosition` structs for this and used them in `Position` as well. We have decided to split `Keyword` enum, but we need them as unified for `PositionComponent` enum. So I didn't split but I can split it if we prefer to change PositionComponent as well.
- If we prefer Keyword enum like this, we can create a SubPosition(or something like this) instead of HorizontalPosition/VerticalPosition enums since only difference is 2 lines in `parse` functions. We can create a `parse_horizontal` and `parse_vertical` instead and a lot of code duplication can be cleared.
- I couldn't find a good way to use HorizontalPosition/VerticalPosition's parse functions in `Position`'s parse function. It is a bit more complicated. I'm open to suggestions :)
- I don't know much about logical keywords so do I need to do something different? I placed some comments where logical keywords are processing.
Any advice about these?
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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.
Expressions with percentages may be negative or positive at computed value time.
So, we can only clamp lengths at computed value time, which is what the other
browsers do.
