Support multi-threaded decoding

Really not an issue, but a request.

Certain codecs, like h264 support multi-threaded decoding. Would this be possible to support?

Here's a quick snip:

///////////////////////////////
pCodecCtx->thread_count = 4;
pCodecCtx->thread_type = FF_THREAD_FRAME;
///////////////////////////////

static int lock_manager(void **mtx, enum AVLockOp op)
    {
        switch(op) 
        {
        case AV_LOCK_CREATE:
            {
            *mtx = new CritSec();
            return 0;
            }
        case AV_LOCK_OBTAIN:
            {
            auto mutex = static_cast<CritSec*>(*mtx);
            mutex->Lock();
            return 0;
            }
        case AV_LOCK_RELEASE:
            {
            auto mutex = static_cast<CritSec*>(*mtx);
            mutex->Unlock();
            return 0;
            }
        case AV_LOCK_DESTROY:
            {
            auto mutex = static_cast<CritSec*>(*mtx);
            delete mutex;
            return 0;
            }
        }
        return 1;
    }
///////////////////////////////
av_lockmgr_register(lock_manager);

Start by locating the decoding setup and the FFmpeg AVCodecContext configuration referenced in the issue. Review how codec initialization and threading are currently handled, including the proposed lock manager. Done means supported codecs such as H.264 can decode with multiple threads without breaking existing decoding behavior.