Use the more accurate, but slightly slower, gjk ray test by default, instead of the sub-simplex convex cast.
Set the rayCallback.m_flag  btTriangleRaycastCallback::kF_UseSubSimplexConvexCastRaytest to enable the previous approximate/faster ray test.

git-svn-id: http://bullet.googlecode.com/svn/trunk@2686 08e121b0-ae19-0410-a57b-3be3395fd4fd