Preserve buffer form previous runs so it's possible to make a compo of full frame, then draw a border and start tweaking nodes and see updates in that border. Main idea is to make it able to visually compare difference between what was changed inside the border and how frame looked before the tweaks outside of the border. Also implemented Clear Viewer Border in compositor, shortcut it Ctrl-Alt-B. Reviewers: lukastoenne, jbakker CC: venomgfx, sebastian_k Differential Revision: https://developer.blender.org/D582