Allow the ForkLiftDemo to toggle between MLCP and SI solver, using F6 key.

Apply patch for CMake config, see Issue 754 (Issue 753)
Fix a few issue with the MLCP solver: allow split impulse, and fix offset in friction dependencies

src/BulletDynamics/MLCPSolvers/btSolveProjectedGaussSeidel.h

@@ -72,6 +72,7 @@ public:
 					x[i]=hi[i]*s;
 			}
 		}
+		return true;
 	}
 
 };