Ability to disable water culling

Water culling is mostly unreliable in OpenMW, and due to how it works randomly its been rather difficult to test out if it's useful or not (on some hardware) in its current state.

Would be nice to have the ability to entirely disable it via an environment variable, it already has one for its debugging view for example (the debugging view itself is broken btw). Or a setting if having it off turns out to be useful to some people.

The debugging view environment variable for those who are curious: OPENMW_WATER_CULLING_DEBUG=1