ShearingMatrix gives matrices corresponding to shearing with the origin kept fixed.
ShearingMatrix gives matrices with determinant 1, corresponding to area- or volume-preserving transformations.
In 2D, ShearingMatrix turns rectangles into parallelograms. ShearingMatrix effectively slants by angle to the right.
In 3D, ShearingMatrix does the analog of shearing a deck of cards by angle in the direction v, with the cards being oriented so as to have normal vector n.