Auto merge of #17696 - jyc:image-orientation-redux, r=Manishearth

Sync Servo / Gecko image-orientation rounding; pass as enum.

Make Servo's rounding of image-orientation values agree with Gecko's,
and pass orientations directly as an enum instead of as angles.

Depends on a Gecko change to be subsequently landed in m-c.

Were reviewed here: https://reviewboard.mozilla.org/r/155336/

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- [ ] These changes fix [Bugzilla Bug 1355380](https://bugzilla.mozilla.org/show_bug.cgi?id=1355380) (github issue number if applicable).

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