Set proper limit for path finding

The exploration limit for path finding was lowered to some guess
depending on the screen size.

Also the hardcoded 1000 in Avatar was removed.

This fixes #658.
Remove obsolete PATH_MAX_TILES definition
MapCollision:compute_path set the default limit parameter to 0

We're dealing with unsigned data types, so better not have -1.
Merge pull request #78 from stefanbeller/pathfinding_performance

merge intelligent pathfinding limit