Currently the ffmpeg demuxers only work in pull-based mode.

The problem is that the ffmpeg demuxers always request data (we can not push data to them). So this will require creating a new task to continue driving the demuxers, and incoming buffers will be coming from the upstream streaming thread.

Most of this can be done in GstFFMpegProtocol. There is a flag on the protocol to inform gstreamer that our stream is not seekable (so that it request sequential buffers).

We will need a GMutex and a GCond to synchronize those two threads and protect the buffer that will be passed along.
Those incoming buffers will be put into a GstAdapter (with a limited size) and we can wake up the demuxer thread once we have the number of bytes that he requested in _read()

Comments are welcome on the design idea. I don't have time currently to implement this but I will try a draft version if nobody has come up with one before.