Add btGearConstraint, with a demo in Bullet/Demos/ConstraintDemo

Thanks to Dimitris Papavasiliou for the idea.

src/BulletDynamics/ConstraintSolver/btGearConstraint.h

@@ -0,0 +1,56 @@
+/*
+Bullet Continuous Collision Detection and Physics Library
+Copyright (c) 2012 Advanced Micro Devices, Inc.  http://bulletphysics.org
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+
+
+
+#ifndef BT_GEAR_CONSTRAINT_H
+#define BT_GEAR_CONSTRAINT_H
+
+#include "BulletDynamics/ConstraintSolver/btTypedConstraint.h"
+///The btGeatConstraint will couple the angular velocity for two bodies around given local axis and ratio.
+///See Bullet/Demos/ConstraintDemo for an example use.
+class btGearConstraint : public btTypedConstraint
+{
+protected:
+	btVector3	m_axisInA;
+	btVector3	m_axisInB;
+	bool		m_useFrameA;
+	btScalar	m_ratio;
+
+public:
+	btGearConstraint(btRigidBody& rbA, btRigidBody& rbB, const btVector3& axisInA,const btVector3& axisInB, btScalar ratio=1.f);
+	virtual ~btGearConstraint ();
+
+	///internal method used by the constraint solver, don't use them directly
+	virtual void getInfo1 (btConstraintInfo1* info);
+
+	///internal method used by the constraint solver, don't use them directly
+	virtual void getInfo2 (btConstraintInfo2* info);
+
+	virtual	void	setParam(int num, btScalar value, int axis = -1) 
+	{
+		btAssert(0);
+	};
+
+	///return the local value of parameter
+	virtual	btScalar getParam(int num, int axis = -1) const 
+	{ 
+		btAssert(0);
+		return 0.f;
+	}
+
+};
+
+#endif //BT_GEAR_CONSTRAINT_H