ffplay: handle fullscreen status on resize event

In some cases resize events can happen even during fullscreen, so let's pass
the proper flag to SDL_SetVideoMode.

Fixes ticket #3964.

Reported-by: Jeff Dwork <jeff@reggie.com>
Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: factorize decoder functions to generic decoder

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: move pts calculating code to generic decoder

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

* cus/stable:
  ffplay: move pts calculating code to generic decoder
  ffplay: factorize decoder functions to generic decoder
  ffplay: handle fullscreen status on resize event

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