performs 2D convolution of matrices a and b
c = moc_conv2( a, b ) c = moc_conv2( a, b, shape )
2D convolution of matrices a and b
(defalut) returns full 2D convolution
returns central part that is same size as a
returns only those parts of the convolution that are computed without the zero-padded edges
conv2 is most efficient when size(a) > size(b)