Cycles: optimization for 3D viewport border render with heavy scenes, the OpenGL
render of objects could slow things down when redrawing the view each time a new
sample is displayed.

Now it does a partial redraw of the viewport with only the render border area,
skipping OpenGL object drawing while the render is refining.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56052 954f8c5b-7b00-dc11-b283-0030488c597c