rtpenc: Expose the max packet size via an avoption

This allows opting for a lower MTU than what the AVIOContext
indicated, and allows writing into outputs that don't indicate
an MTU at all (such as plain files, which is useful for testing).

This also allows querying for the MTU via the avoption.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc.c

@@ -33,6 +33,7 @@
 static const AVOption options[] = {
     FF_RTP_FLAG_OPTS(RTPMuxContext, flags)
     { "payload_type", "Specify RTP payload type", offsetof(RTPMuxContext, payload_type), AV_OPT_TYPE_INT, {.dbl = -1 }, -1, 127, AV_OPT_FLAG_ENCODING_PARAM },
+    { "max_packet_size", "Max packet size", offsetof(RTPMuxContext, max_packet_size), AV_OPT_TYPE_INT, {.dbl = 0 }, 0, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM },
     { NULL },
 };
 
@@ -109,7 +110,12 @@ static int rtp_write_header(AVFormatContext *s1)
         s->first_rtcp_ntp_time = (s1->start_time_realtime / 1000) * 1000 +
                                  NTP_OFFSET_US;
 
-    s->max_packet_size = s1->pb->max_packet_size;
+    if (s->max_packet_size) {
+        if (s1->pb->max_packet_size)
+            s->max_packet_size = FFMIN(s->max_payload_size,
+                                       s1->pb->max_packet_size);
+    } else
+        s->max_packet_size = s1->pb->max_packet_size;
     if (s->max_packet_size <= 12) {
         av_log(s1, AV_LOG_ERROR, "Max packet size %d too low\n", s->max_packet_size);
         return AVERROR(EIO);