As it turns out, the difference between the snapped local rect and the
unsnapped local rect was not just that the former contained snapped
primitives and the latter contained unsnapped primitives, but also that
the former took into account surface inflation for primitives, the
entire clip chain instead of just the primitive's local clip, and
removal of culled primitives. As such, the picture's rects can be wildly
different, even if snapping has been taken care of earlier, and parts of
WebRender have come to rely upon this more accurate representation of a
picture.
Differential Revision: https://phabricator.services.mozilla.com/D46605
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extra : moz-landing-system : lando
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