Improve occlusion and enable in new Vegas

[mchaptel]

The occlusion manager has some issues.

It uses hardcoded addresses for lod nodes, and has the same issues as the missing elements in the shadowmap.

It would be interesting to see if we could fix it and enable it by making it more generic.

For this purpose, the memory addresses for lod can be added to game.h. this is the new Vegas addresses for lod:

0x11C7C28 water 0x11D8690 objects 0x11D86A8 land 0x11D86BC trees

Also it would be helpful to add culling based on fog? Otherwise maybe based on distance/size