mvi: set framerate

This container does not store timestamps and thus supports CFR only.

libavformat/mvi.c

@@ -87,6 +87,7 @@ static int read_header(AVFormatContext *s)
     ast->codec->bit_rate        = ast->codec->sample_rate * 8;
 
     avpriv_set_pts_info(vst, 64, msecs_per_frame, 1000000);
+    vst->avg_frame_rate    = av_inv_q(vst->time_base);
     vst->codec->codec_type = AVMEDIA_TYPE_VIDEO;
     vst->codec->codec_id   = AV_CODEC_ID_MOTIONPIXELS;