Add demos/zone_plate.png

The zone plate image is a useful test case for image scalers because
it contains all representable frequencies, so any imperfection in
resampling filters will show up as Moire patterns.

This version is symmetric around the midpoint of the image, so since
rotating it is supposed to be a noop, it can also be used to verify
that the resampling filters don't shift the image.

V2: Run the file through OptiPNG to cut the size in half, as suggested
by Siarhei.