add option to terminate PGS constraint solvers based on a least square residual threshold

(for example solverInfo().m_leastSquaresResidualThreshold = 1e-7 and use large m_numSolverIterations
disable sphere-sphere contact cache, it is buggy (some contact point stay in the cache, when sphere penetrates more than total margins)
tweak some gpu demo settings

examples/OpenCL/rigidbody/GpuRigidBodyDemo.h

@@ -31,7 +31,8 @@ public:
 	
 	virtual void renderScene();
 	
-	
+	void resetCamera();
+
 	virtual void stepSimulation(float deltaTime);
 
 	//for picking