Support multiple backgrounds in both servo and stylo
(Commits do not build individually, but split up for review)
These patches make all of the background- properties accept multiple values, and add the layout code to display them.
Still needs some cleanup, and some testing, but it seems to work.
r? @SimonSapin
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12902 typedefs
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Using type alias instead of newtype.
Removing duplicated implementation, it's already inherited.
No more type constructors and foo.0
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Cached element class names in style sharing cache with lazy computation
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Fixes#12912
Added a field to StyleSharingCandidate to lazily compute and cache the class names.
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Merge normal and important declarations in style rules
Have a single Vec instead of two. Fix#3426
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Update selectors to 0.10, with ToCss serialization.
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r? @emilio
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