dashenc: copy stream frame rate to output stream

Leads to setting of DefaultDuration in Matroska muxer based on frame rate instead of timebase.
Fixes playback in Chrome.

Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/dashenc.c

@@ -870,6 +870,7 @@ static int dash_write_header(AVFormatContext *s)
         avcodec_parameters_copy(st->codecpar, s->streams[i]->codecpar);
         st->sample_aspect_ratio = s->streams[i]->sample_aspect_ratio;
         st->time_base = s->streams[i]->time_base;
+        st->avg_frame_rate = s->streams[i]->avg_frame_rate;
         ctx->avoid_negative_ts = s->avoid_negative_ts;
 
         if ((ret = avio_open_dyn_buf(&ctx->pb)) < 0)