Fix variable serialization bug on shorthand alias properties
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-moz-transform were having a problem with variable serialization. It wasn't printing transform's value and transform wasn't printing -moz-transform's if their value was a variable. This PR fixes that.
Reduces stylo -moz-transform failures to 2: https://treeherder.mozilla.org/#/jobs?repo=try&revision=3fd3a57873e5e98dfcb173eef4b13822a66fdf6a
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Generate atom files at build-time
The commits here basically do the following things:
1. move all generated files for gecko into "gecko/generated" so that we can copy all of them around
2. make regen_atoms.py generate file to the out dir rather than in-tree
3. make the build script invoke regen_atoms.py when bindgen feature is enabled
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Fix parsing behavior of text-shadow property
Blur radius should reject negative values. There was already `Shadow` struct for properties like this. `filter: drop-shadow` also has the same syntax with that property. I thought sharing the same code would be better and used Shadow struct for parsing. Converted to SpecifiedTextShadow to get rid of unneccessary fields and to be able to convert its computed value. Maybe it would be better to avoid using `Shadow` or just using `Shadow` but sharing code and avoiding unneccessary fields seems better.
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Make stylo support moz-prefixed cursor values.
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This issue is reported at https://bugzilla.mozilla.org/show_bug.cgi?id=1356072.
We need to support the following moz-prefix properties in stylo:
- -moz-grab
- -moz-grabbing
- -moz-zoom-in
- -moz-zoom-out
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- [X] These changes fix [Bug 1356072](https://bugzilla.mozilla.org/show_bug.cgi?id=1356072).
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- [X] These changes do not require tests because the tests already exist in Gecko.
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