Fix for extractSubset() not copying the region.

library/PolyVoxCore/include/SurfaceMesh.inl

@@ -401,6 +401,8 @@ namespace PolyVox
 	{
 		polyvox_shared_ptr< SurfaceMesh<VertexType> > result(new SurfaceMesh<VertexType>);
 		
+		result->m_Region = inputMesh.m_Region;
+
 		if(inputMesh.m_vecVertices.size() == 0) //FIXME - I don't think we should need this test, but I have seen crashes otherwise...
 		{
 			return result;