make btTransform::getIdentity a static const reference, returning a static, and add getIdentity to btMatrix3x3 and btQuaternion class

Thanks to ejtttje and martijn for the suggestions:
http://code.google.com/p/bullet/issues/detail?id=139

src/LinearMath/btMatrix3x3.h

@@ -192,6 +192,15 @@ class btMatrix3x3 {
 					 btScalar(0.0), btScalar(1.0), btScalar(0.0), 
 					 btScalar(0.0), btScalar(0.0), btScalar(1.0)); 
 		}
+
+		static const btMatrix3x3&	getIdentity()
+		{
+			static const btMatrix3x3 identityMatrix(btScalar(1.0), btScalar(0.0), btScalar(0.0), 
+					 btScalar(0.0), btScalar(1.0), btScalar(0.0), 
+					 btScalar(0.0), btScalar(0.0), btScalar(1.0));
+			return identityMatrix;
+		}
+
   /**@brief Fill the values of the matrix into a 9 element array 
    * @param m The array to be filled */
 		void getOpenGLSubMatrix(btScalar *m) const 

src/LinearMath/btQuaternion.h

@@ -275,6 +275,12 @@ public:
 		}
 	}
 
+	static const btQuaternion&	getIdentity()
+	{
+		static const btQuaternion identityQuat(btScalar(0.),btScalar(0.),btScalar(0.),btScalar(1.));
+		return identityQuat;
+	}
+
 	SIMD_FORCE_INLINE const btScalar& getW() const { return m_floats[3]; }
 
 	

src/LinearMath/btTransform.h

@@ -190,11 +190,10 @@ public:
 	btTransform operator*(const btTransform& t) const;
 
   /**@brief Return an identity transform */
-	static btTransform	getIdentity()
+	static const btTransform&	getIdentity()
 	{
-		btTransform tr;
+		static const btTransform identityTransform(btMatrix3x3::getIdentity());
-		tr.setIdentity();
+		return identityTransform;
-		return tr;
 	}
 	
 private: