Store link of equivalent tiles

```
Some tiles (eg. water) are completely equal for different coordinates.
These should be stored only once in the cache.

Tasks:
- find tiles that are probably used more than once
- create link in tile db instead of storing a new tile
```

Original issue reported on code.google.com by `ThyMythos@googlemail.com` on 7 Nov 2009 at 12:25
