Modified floor item rendering. Fixes issue #2.

- Modified the way floor items are rendered: Render the full floor sprite for sections not obscured by any building item, otherwise render only the ceiling. This allows for full window transparency in sky lobbies. [Floor.h, Floor.cpp, Game.cpp]
- Previous workarounds in Sky and Lobby removed since modified floor rendering no longer requires it. [Sky.h, Sky.cpp, Lobby.h, Lobby.cpp]

Bugfixes
--------
- Prevent construction of anything below floor B9, except the Metro Station. [Game.cpp]
- Prevent construction of Metro Station above ground. [Game.cpp]