prepare to re-enable Gwen user interface (OpenGL_DebugFont) for OpenGL2.x

prepare to add improved btGeneric6DofSpringConstraint

src/BulletDynamics/ConstraintSolver/btTypedConstraint.h

@@ -44,6 +44,7 @@ enum btTypedConstraintType
 	D6_SPRING_CONSTRAINT_TYPE,
 	GEAR_CONSTRAINT_TYPE,
 	FIXED_CONSTRAINT_TYPE,
+	D6_SPRING_2_CONSTRAINT_TYPE,
 	MAX_CONSTRAINT_TYPE
 };