brush rendering is now completely client-side.
the server only receives edits the client would like to do, in the form of PNG images of chunks, that are then composited onto the wall
known issue: it is possible to brush up against the current 256 chunk edit limit pretty easily.
I'm not sure it can be solved very easily though. the perfect solution would involve splitting up the interaction into multiple edits, and I tried to do that, but there's a noticable stutter for some reason that I haven't managed to track down yet.
so it'll be kinda crap for the time being.
