Note 1: If you go to a render slot previously rendered and change
something in the compositing the buffer will still vanish.
This is an old bug, T44181, and not addressed here
(I'm basically just fixing the regression introduced with multiview)
Note 2: I have a work in progress patch to get rid of
RenderResult->rectf/rect32/rectz entirely. It still not working, and we
should have a working code base before doing refactoring anyways.
ecb3362a24