Originally I wanted to get rid of RenderResult->rect* entirely, but it's
convenient to have for temporary structs.
This patch makes sure they are used only when really needed, which
should help clearing the code out.
(they are needed when using RE_AcquireResultImage() - which produces a
RenderResult with no RenderView)
Reviewers: sergey
Differential Revision: https://developer.blender.org/D1270
3e6a66b9de