opencv/cxcore.go
func Subtract(src1, src2, dst *IplImage) {
	SubtractWithMask(src1, src2, dst, nil)
}
func Subtract(src1, src2, dst *IplImage) {
	SubtractWithMask(src1, src2, dst, nil)
}
// Calculates the per-element difference between two arrays.
//   dst = src1 - src2