Implement background downloads

Implement background downloading for most resource files, such as 3D world textures. Make the game start nearly immediately upon connecting, even for the first connection, by downloading only the source files and the most indispensable assets to get started.

• Have placeholders such as generic and/or low textures while downloading.
• Apply each download as soon as their download is finished.
• Download files in order of importance.
• Save downloaded files on the client.