Clear VFB and region render behaviour

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.

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