demo: anti-alias example using low-pass filter

[johnnychen94]

In the docstring of imresize:

This interpolates the values at sub-pixel locations. If you are shrinking the image, you risk aliasing unless you low-pass filter img first.

Yet we don't have a good showcase to tell potential users how this can be done.

Typically, we just apply a low-pass Gaussian filter to it. However, the example in the docstring is not very illustrative as it creates a over-blurred result:

using ImageFiltering
σ = map((o,n)->0.75*o/n, size(img), sz)
kern = KernelFactors.gaussian(σ)
imgr = imresize(imfilter(img, kern, NA()), sz)