Support 'word-spacing' for geckolib
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style: Add support for parsing and serialization of <basic-shape>s
Still WIP: I still need to use this somewhere and make serialization minimal. I'm not sure if I should do either in this PR.
The only other browser that handles basic shapes doesn't serialize correctly either (https://bugzilla.mozilla.org/show_bug.cgi?id=1290864), so that's not something we need to get done now.
As far as using this somewhere, I have the following options:
- Merge this now, work on using it in stylo in a followup.
- Just write extensive unit tests for parsing/serialization for now (stylo in a followup)
- Use this for clip-path in stylo only (which I intend to do anyway, just not sure if I should do it in this PR)
- Use this for clip-path in Servo (I'd rather not do this; this would be a huge change requiring a lot more layout knowledge than I currently have)
Thoughts? Review? @SimonSapin @bholley
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Add support for background-position in geckolib.
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r? @Manishearth
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