Add CDG demuxing/decoding support

Submitted by John Carr `@johncarr`

Using ggst-ffmpeg 0.10.11 and gst 0.10.32.

I am using a sample from:

Totem is unable to open it.

I attempted to write a gst-launch pipeline and that yielded an Internal Data Error.

I used:

gst-launch-0.10 --gst-debug ffmpeg:5 filesrc location=/tmp/NobodyKnowsYou.cdg ! ffdemux_cdg ! ffdec_cdgraphics ! ximagesink

The error seems to be coming from gst_ffmpegdata_peek which claims it is pulling 21600 bytes then fails with "Got -3 (unexpected) return result 0".