ao_null: add --ao-null-format option for debugging

Helpful especially to test spdif fallback and so on.

DOCS/man/ao.rst

@@ -198,6 +198,9 @@ Available audio output drivers are:
         If not empty, this is a ``,`` separated list of channel layouts the
         AO allows. This can be used to test channel layout selection.
 
+    ``--ao-null-format``
+        Force the audio output format the AO will accept. If unset accepts any.
+
 ``pcm``
     Raw PCM/WAVE file writer audio output
 

audio/out/ao_null.c

@@ -58,6 +58,7 @@ struct priv {
     int outburst;       // samples
 
     struct m_channels channel_layouts;
+    int format;
 };
 
 static void drain(struct ao *ao)
@@ -88,6 +89,9 @@ static int init(struct ao *ao)
 {
     struct priv *priv = ao->priv;
 
+    if (priv->format)
+        ao->format = priv->format;
+
     ao->untimed = priv->untimed;
 
     struct mp_chmap_sel sel = {.tmp = ao};
@@ -241,6 +245,7 @@ const struct ao_driver audio_out_null = {
         OPT_FLAG("broken-eof", broken_eof, 0),
         OPT_FLAG("broken-delay", broken_delay, 0),
         OPT_CHANNELS("channel-layouts", channel_layouts, 0),
+        OPT_AUDIOFORMAT("format", format, 0),
         {0}
     },
     .options_prefix = "ao-null",