Previously there was very limited texture variations
as a given texture index was used for both the wall
and ceiling. Now these can be specified separately,
allowing for more variation in both STG defined and
random buildings.
Requires simgear commit 053bda26a43314a91b01b08cd4617da82f7ab807
Replace random buildings with one using proper instancing.
Also implement more control over rendering of random buildings.
see README.scenery for details.