ffplay: simplify early frame drop code

Also never early frame drop the first frame after a flush.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: calculate last frame duration from vp->pts instead of frame_last_pts

Also do not update current pts on dropped frames, it is no longer necessary.

Fixes regression part of ticket #2507.

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

* cus/stable:
  ffplay: calculate last frame duration from vp->pts instead of frame_last_pts
  ffplay: simplify early frame drop code

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