* modules/codec/ffmpeg/video_filter.c, include/vlc_filter.h:

* modules/codec/ffmpeg/video_filter.c, include/vlc_filter.h:
  - chroma + resizing video filter (using the filter_t architecture).
* modules/codec/ffmpeg/*:
  - cleanup + small updates.
* modules/codec/speex.c, theora.c, vorbis.c:
  - got rid of pf_header() in the encoder.
  - store the headers in fmt_out.p_extra (this will break the ogg muxer for now).
* modules/codec/libmpeg2.c, modules/codec/ffmpeg/video.c:
  - added a p_dec->b_pace_control field to signal if the decoder is allowed to drop frames.
* modules/stream_out/transcode.c:
  - heavy cleanup.
  - re-use video decoder modules and got rid of the duplicated ffmpeg video decoder.
  - use video filters for chroma conversion and resizing.
  (a few things are broken now like deinterlacing but I'll repair them asap).