Add shadowing demos

[kpreid]

Add to the default world generation objects objects which produce significant shadows. This is tricky because of the high amount of diffuse reflection.

Possibilities:

• Make the outside illumination directional, so that any wall/bump casts a shadow. (Then add walls or bumps.)
• Opaque walls in city worldgen, so that there are less sunny rooms.
• An underground-caves or indoor world gen type.
• Trees with wide canopies.