Applies the specified shear vector to this PMatrix object by prepending the shear transformation.
The transformation applied in this method is a pure shear only if one of the parameters is 0. Applied to a rectangle at the origin, when the shearY factor is 0, the transformation moves the bottom edge horizontally by shearX times the height of the rectangle. When the shearX factor is 0, it moves the right edge vertically by shearY times the width of the rectangle. Caution is in order when both parameters are nonzero, because the results are hard to predict. For example, if both factors are 1, the transformation is singular (hence noninvertible), squeezing the entire plane to a single line.