Clear VFB and region render behaviour

[romullus]

When rendering with area to render: region and clear vfb inbetween renders is selected, whole vfb is erased before render. I think in such case more desirable would be erasing of region only.

That's how it worked back in a days when clear vfb wasn't broken IIRC.

[Ondra]

the solution is to disable "clear vfb". This behavior is consistent with the rest of the cases, so we would like to keep it.