* introducing dct248 into the DSP context.
   * simple/accurate implementation of dct248
   * DV encoding now supports 2-4-8 DCT
   * DV encoding gets a bit faster (but still miles away
     from what I think it could do)
   * misc. DV codec cleanups


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@2425 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b