Font feature settings
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Implement parsing/serialization for font-feature-settings
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Fix radial gradient's <size>/<ending-shape> parsing
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Parsing now handles sizes and shapes in various order.
I had to delete `EndingShape`'s parse implementation and mix it with `Position` in `parse_radial`. It became a bit complicated to read but I couldn't make it simpler.
r? @Manishearth
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