I've decided to fix this in a very explicit way. The only "magic" part that's
left is how we decide that the AGR of the perspective item is outside the
scrolled frame (and I'm not sure myself how that works).
I didn't want to change what scroll clips we set on what items, because the
scroll clip really belongs on the perspective item, because that's the item
that needs to be clipped, and it should also be the item that should be
scrolled if it weren't for the fact that APZ wouldn't know that it should
apply the perspective transform before the APZ transform.
MozReview-Commit-ID: BBw8VPohQI4
--HG--
extra : rebase_source : 9e11441af2b5dab93a5908236e153fc6cd2c3d8b
extra : histedit_source : 7b3dd75afa92346856765284be48df5618821326
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