Removes the four generic PI_imp_ guards that had already been merged into data recently. They have the no longer used script T_ScNpc_Mw_Guard, which wouldn't have done them any good for use in PI in the first place.
They will be re-created as part of the PI main file, so there is no need to fix them, just delete them.
4 new BM tree variants (2 snowy, 2 non-snowy) for VM, using the dead pine needle texture used for fallen branches. These should fit better with the region's colour palette, and show that western plants struggle to survive inside Morrowind's border. Can be used alongside the BM tree stumps, branches, etc.
