m_config: discourage mp_read_option_raw()

This function is dangerous, because it disables the already basic/week
type checking the option system has at all. I'm tend towards thinking
that all of its uses should be replaced.

options/m_config.h

@@ -338,6 +338,7 @@ void *mp_get_config_group(void *ta_parent, struct mpv_global *global,
 // Read a single global option in a thread-safe way. For multiple options,
 // use m_config_cache. The option must exist and match the provided type (the
 // type is used as a sanity check only). Performs semi-expensive lookup.
+// Warning: new code must not use this.
 void mp_read_option_raw(struct mpv_global *global, const char *name,
                         const struct m_option_type *type, void *dst);