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.