r5404 /src/mame/video/snes.c: [SNES]: Fixed the x scrolling wrap around bug when hscroll == 0

Symbolic snapshot (before it was corrupted on the right edge of the screen by just doing a couple of steps forward):
[Linked Image from mamedev.emulab.it]

Actually anything that scrolls in horizontal benefits from this one, I've honestly got an headache from this one (tried to fix it since day one).