Add backface culling and option to keep unflipped hit normal in case a ray hits a back-facing triangle.

Usage: set RayResultCallback.m_flags to kF_FilterBackfaces, optionally combined with kF_KeepUnflippedNormal.
Thanks Andy O'Neil for the patch!

Remove the force_inline for some internal constraint solver methods, it makes re-use easier.
Workaround/avoid MSVC 2005 compiler error in LibXML/trionan.c

src/BulletCollision/CollisionDispatch/btCollisionWorld.h

@@ -186,6 +186,8 @@ public:
 		btCollisionObject*		m_collisionObject;
 		short int	m_collisionFilterGroup;
 		short int	m_collisionFilterMask;
+      //@BP Mod - Custom flags, currently used to enable backface culling on tri-meshes, see btRaycastCallback
+      unsigned int m_flags;
 
 		virtual ~RayResultCallback()
 		{
@@ -199,7 +201,9 @@ public:
 			:m_closestHitFraction(btScalar(1.)),
 			m_collisionObject(0),
 			m_collisionFilterGroup(btBroadphaseProxy::DefaultFilter),
-			m_collisionFilterMask(btBroadphaseProxy::AllFilter)
+			m_collisionFilterMask(btBroadphaseProxy::AllFilter),
+         //@BP Mod
+         m_flags(0)
 		{
 		}