Textures used to be unloaded as soon as they were unreferenced.
The TextureMgr cache keeps an active reference to prevent load-unload-load
cycles so it only unloads things when explicitly asked.
This does require some more peak VRAM during map/tileset transitions
especially, since old textures are unloaded only after the next map
has been loaded, but the shortened load times should be worth it.
Major code/logic cleanups + it has a multithreaded batch mode now.
The VFS code doesn't like multiple threads at all,
so for now there's a big lock in front of where it matters.
Changed precacher, map/tileset, and worldmap loading to batched mode.
Still TODO:
- fix broken mod preview images
- reloading resources on debug key
- make mod recache entirely unnecessary
- actually drop resources when no longer needed
