Pthreads custom wrapper

Hi @sbc100 @kripken

following our conversation on another issue https://github.com/emscripten-core/emscripten/issues/14312#issuecomment-854913896 , I am now building the ffmpeg with `-s ENVIRONMENT=worker -s BUILD_AS_WORKER=1`, on JS side I spin up my custom worker.js (that creates the module) and calls `module._main()`. I am able to call main, but I need to call it with pthread wrapper I guess.

When calling `module._main(argc, argv);`, I see console logging `Tried to spawn a new thread, but the thread pool is exhausted.` so I increased pool size `-s PTHREAD_POOL_SIZE=16`, but event after log errors are gone the further process / logs hangs and is never finished. So my guess I need to properly wrap `main()`?

I wonder if this is possible on JS side or needs to be done in C. I tried to copy over https://github.com/emscripten-core/emscripten/blob/bdc97fe0de1daf9e66aea4fa1d30b3dc3bd907c3/system/lib/pthread/library_pthread.c#L935-L951 into my ffmpeg.c where main() exists, but obviously that throws compiler errors. I am not a C developer, so if possible, I would like to handle as much as possible on JS side.

I have looked into https://github.com/emscripten-core/emscripten/blob/2.0.24/src/library_pthread.js if there is anything that would guide me but no luck so far.