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
a51aeedade