/* scroll in two steps of half tile size to make things smoother */
BlitBitmap(drawto_field, window, fx, fy, SXSIZE, SYSIZE, SX, SY);
/* scroll in two steps of half tile size to make things smoother */
BlitBitmap(drawto_field, window, fx, fy, SXSIZE, SYSIZE, SX, SY);