Auto merge of #17654 - jyc:image-orientation, r=Manishearth

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

Has been reviewed: https://reviewboard.mozilla.org/r/155336/

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Bugzilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1355380

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