Detect soft 2:3 pulldown (telecine) files and decode them at 23.976.

DVD rips from NTSC DVDs are sometimes (always?) encoded using
soft 2:3 pulldown.  The video frames are actually 23.976 but
FFmpeg detects them as 29.97.  With the current approach of the video
decoder ignoring most PTSs and assuming a constant frame rate
it is vital that the file contains the number of frames per second
that the detected frame rate predicts.

This fixes large sync errors with NTSC DVD rips (#1790).