Blender Git Loki

Operator to define render border from rendered image editor

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