Image shear is implemented with both rasterops and linear interpolation.
The rasterop implementation is faster and has no constraints on image
depth. We provide horizontal and vertical shearing about an
arbitrary point (really, a line), both in-place and from source to dest.
The interpolated shear is used on 8 bpp and 32 bpp images, and
gives a smoother result. Shear is used for the fastest implementations
of rotation.