func filter2(pix []byte, level, index, iStep, jStep int) {
	for n := 16; n > 0; n, index = n-1, index+iStep {
		p1 := int(pix[index-2*jStep])
		p0 := int(pix[index-1*jStep])
		q0 := int(pix[index+0*jStep])
		q1 := int(pix[index+1*jStep])
		if abs(p0-q0)<<1+abs(p1-q1)>>1 > level {
			continue
		}
		a := 3*(q0-p0) + clamp127(p1-q1)
		a1 := clamp15((a + 4) >> 3)
		a2 := clamp15((a + 3) >> 3)
		pix[index-1*jStep] = clamp255(p0 + a2)
		pix[index+0*jStep] = clamp255(q0 - a1)
	}
}
filter2 modifies a 2-pixel wide or 2-pixel high band along an edge.
package vp8