* modules/codec/ffmpeg/*: ported the ffmpeg audio and video decoders to the new api.
   Isolated the video postprocessing routines in postprocess.c

* modules/codec/ffmpeg/encoder.c, modules/codec/vorbis.c, modules/stream_out/transcode.c,
   include/vlc_block.h, include/vlc_codec.h: extracted the encoders out of transcode.c.
   transcode now uses encoder plugins (currently ffmpeg, vorbis and theora).

PS: transcoding is currently a bit broken but I'll fix that ASAP.