Added Height Field Fluid Demo to Bullet. All code stored in the Demos/HeightFieldFluidDemo directory for now.

Please see HfFluidDemo.cpp for examples of how to use the height field fluid along with buoyant collision shapes.

The implementation is still lacking in my ways:
1) Need to complete more collision algorithms for buoyant collision shapes
2) Support compound buoyant shapes
3) The buoyancy model isn't that great
4) Fluid volume can be lost over time

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -114,10 +114,11 @@ public:
 	{
 		CO_COLLISION_OBJECT =1,
 		CO_RIGID_BODY,
-		CO_SOFT_BODY,
 		///CO_GHOST_OBJECT keeps track of all objects overlapping its AABB and that pass its collision filter
 		///It is useful for collision sensors, explosion objects, character controller etc.
-		CO_GHOST_OBJECT
+		CO_GHOST_OBJECT,
+		CO_SOFT_BODY,
+		CO_HF_FLUID
 	};
 
 	SIMD_FORCE_INLINE bool mergesSimulationIslands() const