Because DTS values aren't passed through decoders, they tend to be
inaccurate if decoder delay doesn't match what was expected by the encoder.

In particular this improves timestamps for H.264 without num_reorder_frames
set and with -strict 1, which causes DTS to be up to 16 frames ahead of the
picture.

Note that this doesn't really improve any file with very broken PTS/DTS,
since PTS isn't much more accurate in these.

Originally committed as revision 25242 to svn://svn.ffmpeg.org/ffmpeg/trunk