H261 fixing and cleaning:

 -corrected wrong value in mv data
 -set correct mb_type after adjusting index
 -don't use H263 loop filter when the loop filter flag is set but when
  using the H261 encoder
 -use the same unquantizer as H263 (which is optimized btw)
 -removed unused members in H261Context
patch by (Maarten Daniels <maarten.daniels >at< luc >dot< ac >dot< be>)

regression test checksum update by me

Originally committed as revision 3669 to svn://svn.ffmpeg.org/ffmpeg/trunk