lavfi/avfilter: add an "auto" constant to the threads option

Analogous to the same constant in avfiltergraph and avcodec.
Cf. f599ae88c2.

libavfilter/avfilter.c

@@ -652,7 +652,8 @@ static const AVOption avfilter_options[] = {
         { "slice", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AVFILTER_THREAD_SLICE }, .flags = FLAGS, .unit = "thread_type" },
     { "enable", "set enable expression", OFFSET(enable_str), AV_OPT_TYPE_STRING, {.str=NULL}, .flags = TFLAGS },
     { "threads", "Allowed number of threads", OFFSET(nb_threads), AV_OPT_TYPE_INT,
-        { .i64 = 0 }, 0, INT_MAX, FLAGS },
+        { .i64 = 0 }, 0, INT_MAX, FLAGS, .unit = "threads" },
+        {"auto", "autodetect a suitable number of threads to use", 0, AV_OPT_TYPE_CONST, {.i64 = 0 }, .flags = FLAGS, .unit = "threads"},
     { "extra_hw_frames", "Number of extra hardware frames to allocate for the user",
         OFFSET(extra_hw_frames), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, FLAGS },
     { NULL },