This optimization already exists in mozilla-central but needs to be reimplemented for the refactoring. The nice side of this is that once implemented for image bridge, it will be usable for other non-image bridge cases as well (and also that it is buggy on m-c so reimplementing it isn't that bad).

Implemented and pushed to the gfx branch. I did not re-implement the the shared image pool, so we may want to do it as a followup if it turns out to cause a significant performance regression with video elements in Fennec (I am not convinced that it will).