mpegvideo_enc: force encoding delay of at least 1 frame when low_delay=0

This allows the following commit to extrapolate better dts for the first
frame. Pts difference between the first two frames is reused as the
difference between pts and dts of the first frame.

libavcodec/mpegvideo_enc.c

@@ -890,7 +890,8 @@ static int load_input_picture(MpegEncContext *s, AVFrame *pic_arg)
     AVFrame *pic = NULL;
     int64_t pts;
     int i;
-    const int encoding_delay = s->max_b_frames;
+    const int encoding_delay = s->max_b_frames ? s->max_b_frames :
+                                                 (s->low_delay ? 0 : 1);
     int direct = 1;
 
     if (pic_arg) {