[AD] set_difference_blender patch

[Jon R. <wo...@cc...>]

Anyone mind if I apply this patch to set_difference_blender for 4.4? It
clamps the lowest value to 0 instead of using ABS to wrap around. Heres
a post on a.cc that no one responded to:
http://www.allegro.cc/forums/thread/606235

I can add an example program if someone wants.