Make MouseJoint:getBodies() return only one body (fixes #1179)

Override Joint's Body getters in MouseJoint, to make the second (user-supplied) body the first body returned, and return nil for the second (internal) body

src/modules/physics/box2d/MouseJoint.h

@@ -95,6 +95,9 @@ public:
 	 **/
 	float getDampingRatio() const;
 
+	virtual Body *getBodyA() const;
+	virtual Body *getBodyB() const;
+
 private:
 	// The Box2D MouseJoint object.
 	b2MouseJoint *joint;