ext/ffmpeg/gstffmpegdemux.c: Add Tag support and keyframe seeking (for those formats where ffmpeg actually fills in t...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_init),
(gst_ffmpegdemux_close), (gst_ffmpegdemux_handle_seek),
(gst_ffmpegdemux_add), (my_safe_copy), (gst_ffmpegdemux_read_tags),
(gst_ffmpegdemux_open), (gst_ffmpegdemux_loop):
Add Tag support and keyframe seeking (for those formats where ffmpeg
actually fills in the index).
* ext/ffmpeg/gstffmpegprotocol.c: (gst_ffmpegdata_peek),
(gst_ffmpegdata_read), (gst_ffmpegdata_seek):
Add support for size querying.