Fixed multiplayer render bug and more

- added getCollidingPlayer() to physics
- items check for the colliding player instead of the local player
- level objects will do this in the next commit
- added getContainingInventory() to Item class
- refactored MeleeWeapon to support multiple players
- fixed platforms not rendering client-side
- fixed incorrect offset values being sent to clients
- added getParentPlayer() to Inventory class