options: make legacy hacks for AFs/VFs more explicit

This means that AOs/VOs with no options set do not take the legacy
option parsing path, but instead report that they have no options.

audio/filter/af.c

@@ -109,6 +109,7 @@ static bool get_desc(struct m_obj_desc *dst, int index)
 const struct m_obj_list af_obj_list = {
     .get_desc = get_desc,
     .description = "audio filters",
+    .legacy_hacks = true, // many filters have custom option parsing
 };
 
 static bool af_config_equals(struct mp_audio *a, struct mp_audio *b)

core/m_option.c

@@ -2140,7 +2140,7 @@ static int parse_obj_settings(struct bstr opt, struct bstr *pstr,
     }
 
     if (has_param) {
-        if (!desc.options) {
+        if (!desc.options && list->legacy_hacks) {
             // Should perhaps be parsed as escape-able string. But this is a
             // compatibility path, so it's not worth the trouble.
             int next = bstrcspn(*pstr, ",");

core/m_option.h

@@ -125,6 +125,8 @@ struct m_obj_list {
     // Allow unknown entries, for which a dummy entry is inserted, and whose
     // options are skipped and ignored.
     bool allow_unknown_entries;
+    // If object has no options set, assume it parses options on its own.
+    bool legacy_hacks;
 };
 
 // Find entry by name

video/filter/vf.c

@@ -135,6 +135,7 @@ static bool get_desc(struct m_obj_desc *dst, int index)
 const struct m_obj_list vf_obj_list = {
     .get_desc = get_desc,
     .description = "video filters",
+    .legacy_hacks = true, // some filters have custom option parsing
 };
 
 int vf_control(struct vf_instance *vf, int cmd, void *arg)