Refactor the chunk builder to use multithreading on native, but no threads on
wasm, at least until we have web workers or wasm threads. With this change
and the shader fix, chunks now render on Chrome (albeit with no textures).
Another step towards 🕸️ Web support #446.
* Single-threaded chunk builder
* shaders: chunk_frag: consistently enable outputs, fixes GL_INVALID_OPERATION on WebGL