Workaround for sticky convex collisions when using GJK/EPA in combination with very small collision margins.
In some degenerate cases the contact normal is pointing the wrong direction
so fix it now (until we can deal with all degenerate cases in GJK and EPA)
contact normals need to point from B to A in all cases, so we can simply check if the contact normal really points from B to A
We like to use a dot product of the normal against the difference of the centroids, 
once the centroid is available in the API
until then we use the center of the aabb to approximate the centroid

git-svn-id: http://bullet.googlecode.com/svn@2637 08e121b0-ae19-0410-a57b-3be3395fd4fd
Added a gCompoundChildShapePairCallback to filter (enable/disable) child shape collisions for a btCompoundShape.
Thanks to Rich McDaniel for the patch.
See MyCompoundChildShapeCallback in Demos/ConvexDecompositionDemo/ConvexDecompositionDemo.cpp for usage.

git-svn-id: http://bullet.googlecode.com/svn@2638 08e121b0-ae19-0410-a57b-3be3395fd4fd