Expose btManifoldResult::calculateCombinedRestitution and btManifoldResult::calculateCombinedFriction as static member functions, so it can be reused for speculative contacts

Add speculative contact restitution, but disabled by default, not well tested (btDiscreteDynamicsWorld::setApplySpeculativeContactRestitution)
Add --with-double-precision option to premake build system

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.h

@@ -58,6 +58,7 @@ protected:
 	bool	m_ownsIslandManager;
 	bool	m_ownsConstraintSolver;
 	bool	m_synchronizeAllMotionStates;
+	bool	m_applySpeculativeContactRestitution;
 
 	btAlignedObjectArray<btActionInterface*>	m_actions;
 	
@@ -202,6 +203,16 @@ public:
 		return m_synchronizeAllMotionStates;
 	}
 
+	void setApplySpeculativeContactRestitution(bool enable)
+	{
+		m_applySpeculativeContactRestitution = enable;
+	}
+	
+	bool getApplySpeculativeContactRestitution() const
+	{
+		return m_applySpeculativeContactRestitution;
+	}
+
 	///Preliminary serialization test for Bullet 2.76. Loading those files requires a separate parser (see Bullet/Demos/SerializeDemo)
 	virtual	void	serialize(btSerializer* serializer);