Eliminated one loop of going through the drawables in View::GetDrawables(). Instead perform the same work in worker threads.
Early-exit StaticModel::CalculateLodLevels() if only one geometry per batch.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2528 3bd170d8-ddcf-f82c-f583-7a401ccf5608