demos/scale: Added pulldown to choose PIXMAN_FILTER_* value

This is very useful for comparing the results of SEPARABLE_CONVOLUTION
with BILINEAR and NEAREST.

v14: Removed good/best items
v15: Skip filter generation so gnuplot output continues showing previous value

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