Fixed loading-screen flicker in multiplayer.

Flicker was caused because ProgressBarDisplay class was meant to take over rendering. In multiplayer, it ran in the middle of the main render loop instead -- so both the map and the progress bar kept rendering over each other. Now, the network code inside Minecraft.tick() will keep reading from the network non-stop from the moment LEVEL_INIT is received and until the whole map is loaded. Maps actually load slightly faster now!