Slides one edge of an image along the X or Y axis, creating a parallelogram.
   An X direction shear slides an edge along the X axis, while a Y direction
   shear slides an edge along the Y axis.  The amount of the shear is controlled
   by a shear angle.  For X direction shears, x_shear is measured relative to
   the Y axis, and similarly, for Y direction shears y_shear is measured
   relative to the X axis.  Empty triangles left over from shearing the image
   are filled with the background color.