showwaves: fix PTS computation.

libavfilter/avf_showwaves.c

@@ -202,7 +202,10 @@ static void filter_samples(AVFilterLink *inlink, AVFilterBufferRef *insamples)
                                     outlink->w, outlink->h);
             outpicref->video->w = outlink->w;
             outpicref->video->h = outlink->h;
-            outpicref->pts = insamples->pts;
+            outpicref->pts = insamples->pts +
+                             av_rescale_q((p - (int16_t *)insamples->data[0]) / nb_channels,
+                                          (AVRational){ 1, inlink->sample_rate },
+                                          outlink->time_base);
             outlink->out_buf = outpicref;
             linesize = outpicref->linesize[0];
             memset(outpicref->data[0], 0, showwaves->h*linesize);