ext/ffmpeg/gstffmpegdec.c: Separate buffer handling from actual decoding, handle pending frames (for B-frames and so ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_frame),
(gst_ffmpegdec_chain):
Separate buffer handling from actual decoding, handle pending
frames (for B-frames and so on) so we catch the last frames
before EOS, prepare for some timestamp changes to make playback
of movies with B-frames smoother (not done yet).