More efficient billboards
Currently Billboards in NIF files use one draw call per billboard. This is a little tricky to optimize because the billboards are defined as NiBillboardNode + trimesh child, where the trimesh could be a non-quad mesh in principle. Rules out using Ogre::BillboardSet.
(RM-2366 from redmine: created on 2015-02-12 by scrawl ., , relates #2187 (closed))