Underwater Fixes

- Caustics now respects the passed in water plane rather than assume
that the water plane is on the XY plane.  This allows for caustics to
work for steep rivers.
- Fixed lerp() warning in underwaterFogP.hlsl.
- Cleaned up turbulence and caustics PostEffect scripts to remove unused
items.
- Caustics are now enabled and disabled based on the control object
being underwater, just like turbulence.  Moved this code to fog.cs to be
with the underwater postFX.