af: improve filter chain setup retry limit

af_reinit() is responsible for inserting automatic conversion filters
for channel remixing, format conversion, and resampling. We don't
require that a single filter can do all these (even though
af_lavrresample does nearly all of this, sometimes af_format has to be
used instead for format conversions). This makes setting up the chain
more complicated, and a way is needed to prevent endless appending of
conversion filters if a conversion is not possible.

Until now, this used a stupidly simple yet robust static retry limit to
detect failure. This is perfectly fine, and the limit (20) was good
enough to handle about ~5 filters. But with more filters, and if each
filter requires 3 additional conversion filters, this would fail. So
raise the limit to 4 retries per filter. This is still stupidly simple
and robust, but won't arbitrarily fail if the filter count is too large.

audio/filter/af.c

@@ -329,6 +329,14 @@ static void af_print_filter_chain(struct af_stream *s, struct af_instance *at,
     talloc_free(info);
 }
 
+static int af_count_filters(struct af_stream *s)
+{
+    int count = 0;
+    for (struct af_instance *af = s->first; af; af = af->next)
+        count++;
+    return count;
+}
+
 static const char *af_find_conversion_filter(int srcfmt, int dstfmt)
 {
     for (int n = 0; filter_list[n]; n++) {
@@ -438,9 +446,10 @@ int af_reinit(struct af_stream *s)
     // Start with the second filter, as the first filter is the special input
     // filter which needs no initialization.
     struct af_instance *af = s->first->next;
+    int max_retry = af_count_filters(s) * 4; // up to 4 retries per filter
     int retry = 0;
     while (af) {
-        if (retry >= 20)
+        if (retry >= max_retry)
             goto negotiate_error;
 
         // Check if this is the first filter