So now we know what corners to use in the interpolation, but we now
need to implement the actual filtering. Because we want to implement
this in hardware (which has a bilinear interpolation function built-in), we
seek to find a way to express the 1-corner, 2-corner, 3-corner and 4-corner
filtering cases all in terms of a bilinear interpolation function.
The four corner case is trivial, because we see that it's already a standard bilinear interpolation.