filters: add another dumb helper

Can be used with mp_chain_filters() to combine multiple filters into a
single one. This is a bit silly, but whatever. I'm making it an explicit
separate filter, because it lets the user access mp_filter.ppins against
all conventions.

filters/f_utils.c

@@ -178,6 +178,22 @@ struct mp_filter *mp_bidir_nop_filter_create(struct mp_filter *parent)
     return f;
 }
 
+static const struct mp_filter_info bidir_dummy_filter = {
+    .name = "dummy",
+};
+
+struct mp_filter *mp_bidir_dummy_filter_create(struct mp_filter *parent)
+{
+    struct mp_filter *f = mp_filter_create(parent, &bidir_dummy_filter);
+    if (!f)
+        return NULL;
+
+    mp_filter_add_pin(f, MP_PIN_IN, "in");
+    mp_filter_add_pin(f, MP_PIN_OUT, "out");
+
+    return f;
+}
+
 struct fixed_aframe_size_priv {
     int samples;
     bool pad_silence;

filters/f_utils.h

@@ -71,6 +71,12 @@ bool mp_subfilter_drain_destroy(struct mp_subfilter *sub);
 // A bidrectional filter which passes through all data.
 struct mp_filter *mp_bidir_nop_filter_create(struct mp_filter *parent);
 
+// A bidrectional filter which does not connect its pins. Instead, the user is,
+// by convention, allowed to access the filter's private pins, and use them
+// freely. (This is sometimes convenient, such as when you need to pass a single
+// filter instance to other code, and you don't need a full "proper" filter.)
+struct mp_filter *mp_bidir_dummy_filter_create(struct mp_filter *parent);
+
 // A filter which repacks audio frame to fixed frame sizes with the given
 // number of samples. On hard format changes (sample format/channels/srate),
 // the frame can be shorter, unless pad_silence is true. Fails on non-aframes.