Minor bug fixes

Minor bug fixes

Thanks to all who gave a feedback to this modification. I read them and tried to implement them and want to tell you what I changed in the new version:

- Fixed the grass spreading all around on many levels. Now the grass densitiy is normal and looks fine
- Improved some wave designs and changed monster spawns
- Fixed some visual background bugs in level 9
- Fixed a door in level 11 which stayed open at the beginning of the level
- Fixed the connection from level 11 & 12, so that level 12 is loading correctly
- Fixed wrong wave design in level 13, which spawned way too many enemies at the same time

I was not able to reproduce multiplayer bugs, especially visual bugs with weapons picked up in a level and sprinting, although I grabbed a friend and started my own server. Please make sure to deactivate all other mods and do not use modded player models. If errors still occur, please describe them using as much information as possible, for example level name, used player model, game settings and so on.