This way it is possible to define render border directly from the image editor, which is useful for at least three things: - If viewport is really optimized out (simplified etc) then it might be hard to guess which exact area you're mainly interested now. - No need to switch to the viewport to do render border tweaks, could be useful when doing compositing. - If one need to look at particular pixel(s) which is real handy for debugging render engines (both Cycles and BI). Reviewers: campbellbarton, venomgfx Reviewed By: venomgfx Differential Revision: https://developer.blender.org/D820