ffplay: remember last window dimensions

After this change the dimensions of single image videos will be remembered when
coming back from full screen. The issue was mentioned in ticket #2174.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: move up pause functions

No change in functionality.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: step to next frame if paused when seeking

Signed-off-by: Marton Balint <cus@passwd.hu>