worm 0.92.0623: alpha release; tron, fat lines

- fix left+down bug
  (froze the calc while certain keys were pressed simultaneously)
- Tron game mode in which worms do not shrink (both single- and multiplayer)
- extensible object types (adds 'fat lines' and allows for more)
- default sprite fallback
- very limited release
  (add some padding to indicate future size increase)