Make the PaddingEffect border 1-pixel soft.

Make the PaddingEffect border 1-pixel soft.

Note that this is an API break; PaddingEffect now does something else
from what it used to do before when it comes to fractional offsets.
But I feel this is more useful; it allows PaddingEffect to be used
more efficiently for moving things smoothly around.

Also add a concept of border offset which moves the border around
without changing the pixels; useful if you want the subpixel placement
to be done by ResampleEffect (put the integral offset into top/left
and then move the border by the fractional amount it missed).