Patch v1.8.4

- Fixed LoadEffect() startup crash.
- Fixed crash when unloading a level in a maximized window.
- Fixed save game backups from crashing if a backup already exists.
- Added all 8 co-op levels to co-op survival (Syzygy and Trifecta for 3 players. Desert City, Burner, Chalice, Double-Cross, Reservoir, and At the Brink for 4 players).