Only send users what is in view of them when updating state

[bdjewkes]

Currently the server ends every tick by calling emit('world', game_state); eg, the global world state is broadcast to all clients. There are a few problem with this, but the most immediate ones are:

• every client will know everything about the world! (good luck having a "hide" ability with this model)
• the client needs to know how to translate between "world space" coordinates and "view space" and "screen space" coordinates

Instead of broadcasting global state, entities should have a view range (this can be a global constant to begin with, but could eventually be adjustable per entity). Each frame, instead of emitting a global message, each client would have everything within their view range sent to them