audio: allow more than 20 channel map entries

This could trigger an assertion when using ao_alsa or ao_coreaudio. The
code was simply assuming the number of channel maps was bounded
statically (which was true at first in both AOs).

Fix by using dynamic memory allocation. It needs to be explicitly
enabled by the AOs by setting a temp context, because otherwise the
memory couldn't be freed. (Or at least this seems to be the most elegant
solution.)

Fixes #1306.

audio/chmap_sel.c

@@ -81,14 +81,21 @@ void mp_chmap_sel_add_alsa_def(struct mp_chmap_sel *s)
     }
 }
 
-#define ARRAY_LEN(x) (sizeof(x) / sizeof((x)[0]))
-
 // Add a channel map that should be allowed.
 void mp_chmap_sel_add_map(struct mp_chmap_sel *s, const struct mp_chmap *map)
 {
-    assert(s->num_chmaps < ARRAY_LEN(s->chmaps));
-    if (mp_chmap_is_valid(map))
-        s->chmaps[s->num_chmaps++] = *map;
+    if (!mp_chmap_is_valid(map))
+        return;
+    if (!s->chmaps)
+        s->chmaps = s->chmaps_storage;
+    if (s->num_chmaps == MP_ARRAY_SIZE(s->chmaps)) {
+        if (!s->tmp)
+            return;
+        s->chmaps = talloc_memdup(s->tmp, s->chmaps, sizeof(s->chmaps_storage));
+    }
+    if (s->chmaps != s->chmaps_storage)
+        MP_TARRAY_GROW(s->tmp, s->chmaps, s->num_chmaps);
+    s->chmaps[s->num_chmaps++] = *map;
 }
 
 // Allow all waveext formats in default order.

audio/chmap_sel.h

@@ -26,8 +26,12 @@ struct mp_chmap_sel {
     // should be considered opaque
     bool allow_any, allow_waveext;
     bool speakers[MP_SPEAKER_ID_COUNT];
-    struct mp_chmap chmaps[20];
+    struct mp_chmap *chmaps;
     int num_chmaps;
+
+    struct mp_chmap chmaps_storage[20];
+
+    void *tmp; // set to any talloc context to allow more chmaps entries
 };
 
 void mp_chmap_sel_add_any(struct mp_chmap_sel *s);

audio/out/ao_alsa.c

@@ -279,7 +279,7 @@ static int find_alsa_channel(int mp_channel)
 static bool query_chmaps(struct ao *ao, struct mp_chmap *chmap)
 {
     struct priv *p = ao->priv;
-    struct mp_chmap_sel chmap_sel = {0};
+    struct mp_chmap_sel chmap_sel = {.tmp = p};
 
     snd_pcm_chmap_query_t **maps = snd_pcm_query_chmaps(p->alsa);
     if (!maps)

audio/out/ao_coreaudio.c

@@ -185,7 +185,7 @@ static bool init_chmap(struct ao *ao)
                        &layouts, &n_layouts);
     CHECK_CA_ERROR("could not get audio device prefered layouts");
 
-    struct mp_chmap_sel chmap_sel = {0};
+    struct mp_chmap_sel chmap_sel = {.tmp = p};
     for (int i = 0; i < n_layouts; i++) {
         struct mp_chmap chmap = {0};
         if (ca_layout_to_mp_chmap(ao, &layouts[i], &chmap))