What's new?

Background:

1. Added background functionality for inventory.
2. Added background functionality for dropping items.
3. Added background functionality for collections.

Content:

1. Added most items to the game.

Rendering:

1. Improved sprite rendering algorithm.
   
2. Improved sprite removal algorithm.
3. Fixed an issue that caused sprites to start after they should have been removed.

Other:

1. Simplified UI API.