vo: mark frame lost after seek reset

Reverts a small change made in commit ed9295c. This is needed, because
otherwise mplayer.c/update_video_attached_pic() thinks it never has to
update the picture after initialization. (Maybe there would be more
elegant ways to handle this, but not without adding extra state.)

video/out/vo.c

@@ -289,6 +289,7 @@ void vo_seek_reset(struct vo *vo)
 {
     vo_control(vo, VOCTRL_RESET, NULL);
     vo->frame_loaded = false;
+    vo->hasframe = false;
     mp_image_unrefp(&vo->waiting_mpi);
 }