Reduce tearing/flicker even further -  merge in ajanced or overlapping
updates into a single VGLBitmapCopy() operation, so we don't end up with half
of the object removed (i.e. when the object moves inbetween position A to
position B), while the other half at the pos B drawn in a subsequent
operation. I think this is the best we can do without actual access to
the vertical retrace signal.