Calculate \be using [0..64] value range (like VSFilter)

Calculate \be using [0..64] value range (like VSFilter)

To avoid banding in the output, the full [0..255] value range
is restored before the last \be pass, which then uses the full
range and hides the bands by virtue of being a blur.

With this, our \be finally closely matches VSFilter's.

The only visible difference (other than the lack of banding) is
in clipping: we add proper padding and output the whole blurred
image, while VSFilter does not add any padding and hence clips
the blurred image too early.