Bird AI, fixes

Bird AI:

Birds have been added. In future, they will be the main objective, but for now they are purely decoration.

Bugs:

• Birds may spawn floating. This is because they don't raycast to find a collider when they spawn.
• Birds may fly through solid objects.

Bugfixes:

• Fixed issues with gamma lighting

If you notice performance issues due to the number of birds spawned, let me know and I will do my best to rescale the amount.