libobs: Add obs_source_info.filter_add

If there is filter_remove, it is reasonable to expect that there is also
filter_add. filter_add also enables filters to attach signal handlers
on the parent, and disconnect them in filter_remove.

libobs/obs-source.c

@@ -3132,6 +3132,10 @@ void obs_source_filter_add(obs_source_t *source, obs_source_t *filter)
 
 	blog(LOG_DEBUG, "- filter '%s' (%s) added to source '%s'",
 	     filter->context.name, filter->info.id, source->context.name);
+
+	if (filter->info.filter_add)
+		filter->info.filter_add(filter->context.data,
+					filter->filter_parent);
 }
 
 static bool obs_source_filter_remove_refless(obs_source_t *source,

libobs/obs-source.h

@@ -552,6 +552,14 @@ struct obs_source_info {
 	enum gs_color_space (*video_get_color_space)(
 		void *data, size_t count,
 		const enum gs_color_space *preferred_spaces);
+
+	/**
+	 * Called when the filter is added to a source
+	 *
+	 * @param  data    Filter data
+	 * @param  source  Source that the filter is being added to
+	 */
+	void (*filter_add)(void *data, obs_source_t *source);
 };
 
 EXPORT void obs_register_source_s(const struct obs_source_info *info,