Reference: pmShear
pmShear(pm, x, y) applies a shearing transform to the pixmap pm, and returns the result. x and y are the magnitudes in the horizontal and vertical direction, respectively. Thus, pmShear(pm, 0, 0) is the identity transformation, and pmShear(pm, 0.2, 0) applies a small horizontal shear. It is required that xy ≠ 1.