Major multiplayer improvements

- increased multiplayer stability and reduced disconnects
- added error messages for when connections fail
- instead of sending all the entities in one packet, it sends one per packet
- smoothed client-side rendering by sending the velocity values along with coordinates (so in between updates the client can still move entities)
- client screen GUI updated to match host GUI
- enemies spawn near random player
- fixed bug where defense powerups increase damage dealt to players
- fixed bug where client players could not shoot
- dimmed background texture to make bullets easier to see