convolveWithBox now allows non-square boxes (pass an ImageRef for hwin instead of an int, but int version still exists).
convolveWithBox now allows non-square boxes (pass an ImageRef for hwin
instead of an int, but int version still exists).

Added convolve_gaussian_3, which performs convolution with a 3x3 gaussian
envelope (9 reads, one write per pixel).

oneminot/libcvd,oneminot/libcvd,oneminot/libcvd