Trigger reflow on document.elementsFromPoint
As [suggested](https://github.com/servo/servo/issues/15592#issuecomment-280379805) by @jdm `Document::nodes_from_point` now triggers a reflow.
I added a new reftest that panics with `ERROR:servo: Tried to hit test without a DisplayList` if this patch is not applied.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#15592.
- [X] There are tests for these changes
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Don't serialize shorthand if some but not all its longhands have CSS-wide keyword
This also changes `LonghandsToSerialize` to store references to specified value directly rather than declared value, which significantly simplify many serialization code.
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Shorthand cannot represent CSS-wide keyword in individual longhands, and
thus we cannot serialize longhand with CSS-wide keyword as part of a
shorthand.
Fix border shorthand serialization
Fix border shorthand serialization when sides are not identical. I think I managed to get the serialization to work as expected.
I added a check to ensure that the **border-width**, **border-style** and **border-color** were the same, before performing the serialization.
I'm still a Rust newbie, so if there's a more idiomatic way of doing things (or any critiques in general), please let me know!
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- [X] These changes fix#15395
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Remove the redundant tracked loads for the underlying image request; the
load blockers present in the ImageRequest structure perform the same
duty with better accuracy.
Update cssparser
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https://github.com/servo/rust-cssparser/pull/123
In particular, `match_ignore_ascii_case` now supports the full `match` syntax.
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- [ ] These changes fix #__ (github issue number if applicable).
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