[fix] Small glitches with zeiger_t

[gerw]

1st: zeiger_t doesn't respect the y_off when it marks images dirty. Patch at www.tu-chemnitz.de/~gerw/patches/dirty_offset.patch

2nd: I think, in pak64 there are 2 pixels to much in the grid overlay. They sometimes get stuck at the screen, when moving the cursor (this happens yesterday, but I can't reproduce it now..). See www.tu-chemnitz.de/~gerw/patches/cursor.png