ext/ffmpeg/: Fix up demuxer. Works now for all my files that I tried, even quicktime. I basically hack around the sup...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_src_event),
(gst_ffmpegdemux_src_query), (gst_ffmpegdemux_src_convert),
(gst_ffmpegdemux_open), (gst_ffmpegdemux_loop):
* ext/ffmpeg/gstffmpegprotocol.c: (gst_ffmpegdata_open),
(gst_ffmpegdata_read), (gst_ffmpegdata_write),
(gst_ffmpegdata_seek):
Fix up demuxer. Works now for all my files that I tried, even
quicktime. I basically hack around the super-oversimplistic file
I/O handling in ffmpeg in several ways together, and I also hack
around the fairly annoying EOS-will-pause-the-source-element in
GStreamer itself.
This code is not pretty.