ext/ffmpeg/: Move palette handling over from the decoder to the colorspace conversion plugin (where you would expect ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_pixfmt_to_caps),
(gst_ffmpeg_caps_to_pixfmt):
* ext/ffmpeg/gstffmpegcolorspace.c: (gst_ffmpegcsp_pad_link),
(gst_ffmpegcsp_init), (gst_ffmpegcsp_chain),
(gst_ffmpegcsp_change_state), (gst_ffmpegcsp_register):
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_chain):
Move palette handling over from the decoder to the colorspace
conversion plugin (where you would expect it).