vf_tonemap: Update hdr metadata with the new peak value

Less effective than the approach in vf_tonemap_opencl because there
is no peak detection, but it's still a good idea to implement this.

libavfilter/vf_tonemap.c

@@ -265,6 +265,8 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
 
     av_frame_free(&in);
 
+    ff_update_hdr_metadata(out, peak);
+
     return ff_filter_frame(outlink, out);
 }