demos/scale: Compute filter size using boundary of xformed ellipse

Instead of using the boundary of xformed rectangle, use the boundary
of xformed ellipse. This is much more accurate and less blurry. In
particular the filtering does not change as the image is rotated.

Signed-off-by: Bill Spitzak <spitzak@gmail.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Soren Sandmann <soren.sandmann@gmail.com>