improve rolling friction using anisotropic direction, to avoid resting in an instable position

(for implicit capsule, cylinder and cone shape)
See Bullet/Demos/RollingFrictionDemo for an example

src/BulletCollision/CollisionShapes/btCylinderShape.h

@@ -97,6 +97,13 @@ BT_DECLARE_ALIGNED_ALLOCATOR();
 		return m_upAxis;
 	}
 
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		btVector3 aniDir(0,0,0);
+		aniDir[getUpAxis()]=1;
+		return aniDir;
+	}
+
 	virtual btScalar getRadius() const
 	{
 		return getHalfExtentsWithMargin().getX();