Returns a pixmap with a wind effect.
wind(pm, p, d, f)
pmis a pixmap
pis a real number
dis an integer
fis a real number
wind(pm, p, d, f) returns
pm with a wind effect.
p is the probability that a pixel is “blown away”,
d the average distance a pixel moves (if at all), and
f the fade-out factor (between 0 and 1).
wind(ExampleData("Sally"), 1, 20, 1)