Add stdin stream support.

Many applications stream STDIN to allow ffmpeg to encode from a data source that is not a file, or a data source that will never end (i.e, livestreaming a web cam).

Right now the lib can't support it because stdin read in WASM needs to be blocking (which freezes the thread) and this means the event loop is frozen too so that data that would have been sent via postMessage can't be processed.

Describe the solution you'd like I added stdin buffer support to a fork of the other ffmpeg.js that allows STDIN to be read inside a worker. https://github.com/PaulKinlan/ffmpeg.js/pull/1

It uses SharedArrayBuffers to communicate between the window and the worker, so this limit's it to Chrome on Desktop at the moment. It does however allow ffmpeg to block on stdin and then recieve data from the window asynchronously.