This fixes the jerky video from vob files. However, I am convinced that this
is the wrong solution to the problem. In particular, if you specify an output
frame rate, then it doesn't work correctly. My feeling is that the pts value
ought to be interpolated between those frames where the pts is actually
known. Maybe someone else could do that (change the line that I added,
setting a value equal to AV_NOPTS_VALUE).
Originally committed as revision 1361 to svn://svn.ffmpeg.org/ffmpeg/trunk