add option for slightly more accurate, and slightly slower algorithm for ray versus convex, used in btCollisionWorld::rayTest.

rayResult.m_flags |= btTriangleRaycastCallback::kF_UseGjkConvexRaytest;
See Demos/RaytestDemo/RaytestDemo.cpp for usage
This fixes issue 146, longest outstanding from 2008.

src/BulletCollision/NarrowPhaseCollision/btRaycastCallback.h

@@ -35,7 +35,7 @@ public:
       kF_None                 = 0,
       kF_FilterBackfaces      = 1 << 0,
       kF_KeepUnflippedNormal  = 1 << 1,   // Prevents returned face normal getting flipped when a ray hits a back-facing triangle
-
+	  kF_UseGjkConvexRaytest =  1 << 2,   // Uses a more accurate but slightly slower ray versus convex algorithm
       kF_Terminator        = 0xFFFFFFFF
    };
    unsigned int m_flags;