avfiltergraph: avoid harmful auto audio conversions.

With the current code, an automatically inserted aconvert necessary
for format change would usually convert to mono for no good reason.

The new code will not avoid all conversions, but at least will keep
them among the layouts common to both filters.

libavfilter/avfiltergraph.c

@@ -187,6 +187,7 @@ static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
 {
     int i, j, ret;
     char filt_args[128];
+    AVFilterFormats *formats, *chlayouts, *packing;
 
     /* ask all the sub-filters for their supported media formats */
     for (i = 0; i < graph->filter_count; i++) {
@@ -221,9 +222,13 @@ static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
                     !link->in_packing   || !link->out_packing)
                     return AVERROR(EINVAL);
 
-                if (!avfilter_merge_formats(link->in_formats,   link->out_formats)   ||
-                    !avfilter_merge_formats(link->in_chlayouts, link->out_chlayouts) ||
-                    !avfilter_merge_formats(link->in_packing,   link->out_packing))
+                /* Merge all three list before checking: that way, in all
+                 * three categories, aconvert will use a common format
+                 * whenever possible. */
+                formats   = avfilter_merge_formats(link->in_formats,   link->out_formats);
+                chlayouts = avfilter_merge_formats(link->in_chlayouts, link->out_chlayouts);
+                packing   = avfilter_merge_formats(link->in_packing,   link->out_packing);
+                if (!formats || !chlayouts || !packing)
                     if (ret = insert_conv_filter(graph, link, "aconvert", NULL))
                        return ret;
             }