optimization & bugfix extracted from the 4k line diff between ffmpeg 0.4.7 and http://www.alicestreet.com/ffh263.html
the other parts of the diff where
1. spelling fixes (rejected as only a small part of it could be applied automatically)
2. cosmetics (reindention, function reordering, var renaming, ...) with bugs (rejected)
3. rtp related stuff (rejetced as it breaks several codecs)
4. some changes to the intra/inter decission & scene change detection (quality tests needed first)

Originally committed as revision 2721 to svn://svn.ffmpeg.org/ffmpeg/trunk