fftools/opt_cmdutils: fix printing known channel layouts

Look for the generic "USR" labels instead of "?" to skip channels with no
known names, and actually print the decomposition of standard channel layouts.

Signed-off-by: James Almer <jamrial@gmail.com>

fftools/opt_common.c

@@ -997,7 +997,7 @@ int show_layouts(void *optctx, const char *opt, const char *arg)
            "NAME           DESCRIPTION\n");
     for (i = 0; i < 63; i++) {
         av_channel_name(buf, sizeof(buf), i);
-        if (!strcmp(buf, "?"))
+        if (strstr(buf, "USR"))
             continue;
         av_channel_description(buf2, sizeof(buf2), i);
         printf("%-14s %s\n", buf, buf2);
@@ -1006,11 +1006,14 @@ int show_layouts(void *optctx, const char *opt, const char *arg)
            "NAME           DECOMPOSITION\n");
     while (ch_layout = av_channel_layout_standard(&iter)) {
             av_channel_layout_describe(ch_layout, buf, sizeof(buf));
-            av_channel_name(buf2, sizeof(buf2), i);
             printf("%-14s ", buf);
-            for (i = 0; i < 63; i++)
-                if (av_channel_layout_index_from_channel(ch_layout, i) >= 0)
-                    printf("%s%s", i ? "+" : "", buf2);
+            for (i = 0; i < 63; i++) {
+                int idx = av_channel_layout_index_from_channel(ch_layout, i);
+                if (idx >= 0) {
+                    av_channel_name(buf2, sizeof(buf2), i);
+                    printf("%s%s", idx ? "+" : "", buf2);
+                }
+            }
             printf("\n");
     }
     return 0;