avfilter/af_adelay: add option which changes how unset channels are delayed

Fixes #8032.

doc/filters.texi

@@ -688,6 +688,10 @@ Unused delays will be silently ignored. If number of given delays is
 smaller than number of channels all remaining channels will not be delayed.
 If you want to delay exact number of samples, append 'S' to number.
 If you want instead to delay in seconds, append 's' to number.
+
+@item all
+Use last set delay for all remaining channels. By default is disabled.
+This option if enabled changes how option @code{delays} is interpreted.
 @end table
 
 @subsection Examples
@@ -706,6 +710,12 @@ the first channel (and any other channels that may be present) unchanged.
 @example
 adelay=0|500S|700S
 @end example
+
+@item
+Delay all channels by same number of samples:
+@example
+adelay=delays=64S:all=1
+@end example
 @end itemize
 
 @section aderivative, aintegral

libavfilter/af_adelay.c

@@ -36,6 +36,7 @@ typedef struct ChanDelay {
 
 typedef struct AudioDelayContext {
     const AVClass *class;
+    int all;
     char *delays;
     ChanDelay *chandelay;
     int nb_delays;
@@ -54,6 +55,7 @@ typedef struct AudioDelayContext {
 
 static const AVOption adelay_options[] = {
     { "delays", "set list of delays for each channel", OFFSET(delays), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, A },
+    { "all",    "use last available delay for remained channels", OFFSET(all), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, A },
     { NULL }
 };
 
@@ -163,6 +165,11 @@ static int config_input(AVFilterLink *inlink)
         }
     }
 
+    if (s->all) {
+        for (int j = i + 1; j < s->nb_delays; j++)
+            s->chandelay[j].delay = s->chandelay[i].delay;
+    }
+
     s->padding = s->chandelay[0].delay;
     for (i = 1; i < s->nb_delays; i++) {
         ChanDelay *d = &s->chandelay[i];