Make ChunkFile use "vectors" for storage rather than looking for a size ahead of time. This is so that it can be used for netplay too. Not that it really saves much to get rid of 400 lines of SFML code, but it feels a little silly to have two different mechanisms, and ChunkFile would be more suitable when more types of data need to be sent over the socket soon(tm). Instead of using an actual vector it uses a PWBuffer, which is like a std::vector<u8> but without initialization of elements to 0, avoiding some preexisting slowdown. May as well use that everywhere.