ffplay: avoid code duplication in AVFILTER enabled and disabled case

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add frame duration estimated from frame rate to VideoPicture

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add smarter method for determining video picture duration

- consider it an invalid PTS when the next PTS value is the same as the current one
- in case of invalid or unknown PTS, return vp->duration

This fixes ffplay part of ticket #3005.

Signed-off-by: Marton Balint <cus@passwd.hu>
Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: add smarter method for determining video picture duration
  ffplay: add frame duration estimated from frame rate to VideoPicture
  ffplay: avoid code duplication in AVFILTER enabled and disabled case

Merged-by: Michael Niedermayer <michaelni@gmx.at>