Simulates a pencil sketch. We convolve the image with a Gaussian operator
of the given radius and standard deviation (sigma). For reasonable
results, radius should be larger than sigma. Use a radius of 0 and
Imagick::sketchImage() selects a suitable radius for you. Angle gives the
angle of the blurring motion.
This method is available if Imagick has been compiled against ImageMagick version 6.2.9 or newer.