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/GpuRigidBodyDemoInternalData.h

@@ -32,6 +32,7 @@ struct	GpuRigidBodyDemoInternalData
 	int m_pickGraphicsShapeIndex;
 	int m_pickGraphicsShapeInstance;
 	b3Config m_config;
+	GUIHelperInterface* m_guiHelper;
 
 	GpuRigidBodyDemoInternalData()
 		:m_instancePosOrnColor(0),
@@ -45,7 +46,8 @@ struct	GpuRigidBodyDemoInternalData
 		m_pickGraphicsShapeInstance(-1),
 		m_pickBody(-1),
 		m_altPressed(0),
-		m_controlPressed(0)
+		m_controlPressed(0),
+		m_guiHelper(0)
 
 	{
 	}