ext/ffmpeg/gstffmpeg.c: Enable muxers (only mp4 muxer for now).

ext/ffmpeg/gstffmpeg.c: Enable muxers (only mp4 muxer for now).

Original commit message from CVS:
* ext/ffmpeg/gstffmpeg.c: (plugin_init):
Enable muxers (only mp4 muxer for now).
* ext/ffmpeg/gstffmpegcodecmap.c:
(gst_ffmpeg_formatid_get_codecids), (gst_ffmpeg_caps_to_codecid):
* ext/ffmpeg/gstffmpegcodecmap.h:
Fix a bunch of typos in codec-id lookup (false/true return value
mixup), add a codec-id list retrieval function (because ffmpeg
does not provide one). With that, we can make valid pad templates.
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_init),
(gst_ffmpegmux_connect), (gst_ffmpegmux_loop),
(gst_ffmpegmux_change_state), (gst_ffmpegmux_get_id_caps),
(gst_ffmpegmux_register):
Fix for whatever changed since I last tried this. Works for MP4
muxing.
* ext/ffmpeg/gstffmpegprotocol.c: (gst_ffmpegdata_open),
(gst_ffmpegdata_peek), (gst_ffmpegdata_write),
(gst_ffmpegdata_seek), (gst_ffmpegdata_close):
Update obviously-untested write code...