I Broke Volumetric Fog

I'll try to get it fixed soon....it's a bit frustrating, because it's the second time in the last week that I made a change to X-Plane to try to improve performance, tested it on my hardware, then discovered in-field that it helps some machines but screws up a lot of others.

The fog was a screw-up though...it's broken on the 9600XT and I have one of those.

(I'm not entirely sure what the minimum graphics card for volumetric fog will turn out to be...right now we let you use it no matter how slow it makes the computer, but generally it's been sort of a performance problem...it's just a very expensive algorithm that needs some kind of restructuring.)