fixing 2 small qpel bugs

Originally committed as revision 915 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 788 to svn://svn.ffmpeg.org/ffmpeg/trunk

returning buf_size insetad of 0

Originally committed as revision 767 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 766 to svn://svn.ffmpeg.org/ffmpeg/trunk

 MpegEncContext->opaque MUST NOT be used by the codec, its for the parent program
 using standart values for pict_type
 using (uv)linesize
 handling dropable p-frames like b-frames

Originally committed as revision 763 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 761 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 757 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 744 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 743 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 742 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 736 to svn://svn.ffmpeg.org/ffmpeg/trunk

* generic optimization:
  - enable ALT and ALIGNED stream
  - move "if(const)" out from loops
  - supress duplicated computing
* first attempt to fix problem with strides

Originally committed as revision 734 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 726 to svn://svn.ffmpeg.org/ffmpeg/trunk