fixed mpeg4 time stuff on encoding

mpeg4 b-frame enoding support
removed old, out-commented ratecontrol
reuse motion compensation code between encoding & decoding
prefix newly added global functions with ff_ to reduce namespace polution
b-frame ME (unfinished, but working)
added some comments to mpegvideo.h
do MC on encoding only once if possible
bugs? ;)

Originally committed as revision 403 to svn://svn.ffmpeg.org/ffmpeg/trunk