polyMesh: Stopped re-seeding of RNG on each cell-tree update

This is a quick fix. What actually needs doing is the Random and
cachedRandom classes need rewriting in terms of the random number
functionality in the C++11 STL. These can be initialised/seeded
per-object, which makes this sort of bug go away.

This resolves bug report https://bugs.openfoam.org/view.php?id=2772

src/OpenFOAM/meshes/polyMesh/polyMesh.C

@@ -878,7 +878,7 @@ Foam::polyMesh::cellTree() const
     {
         treeBoundBox overallBb(points());
 
-        Random rndGen(261782);
+        static Random rndGen(261782);
 
         overallBb = overallBb.extend(rndGen, 1e-4);
         overallBb.min() -= point(ROOTVSMALL, ROOTVSMALL, ROOTVSMALL);