Fix zoomed-in slab selection at 4K resolution

[rainlizard]

1. Set your resolution to 3840x2160
2. Go to the bottom right of the map. (so the second half of coordinates)
3. Zoom all the way in.
4. You'll have trouble moving your cursor around on slabs.

The variables just weren't large enough to handle the coordinates. Instead of long they needed to be long long, so I've just used int64_t.