libobs: Keep transition reference counter

The reason to keep a reference counter for transitions is due to an
optimization I'm planning on when calculating transition volumes.  I'm
planning on walking the source tree to be able to calculate the current
base volume of a source, but *only* if there are transitions active,
because the only time that the volume can be anything other than 1.0
or 0.0 is when there are active transitions, which may change the base
volume of a source.

libobs/obs-internal.h

@@ -198,6 +198,8 @@ struct obs_core_data {
 
 	struct obs_view                 main_view;
 
+	volatile long                   active_transitions;
+
 	long long                       unnamed_index;
 
 	volatile bool                   valid;

libobs/obs-source.c

@@ -189,6 +189,9 @@ obs_source_t *obs_source_create(enum obs_source_type type, const char *id,
 
 	blog(LOG_INFO, "source '%s' (%s) created", name, id);
 	obs_source_dosignal(source, "source_create", NULL);
+
+	if (info->type == OBS_SOURCE_TYPE_TRANSITION)
+		os_atomic_inc_long(&obs->data.active_transitions);
 	return source;
 
 fail:
@@ -223,6 +226,9 @@ void obs_source_destroy(struct obs_source *source)
 	if (!source)
 		return;
 
+	if (source->info.type == OBS_SOURCE_TYPE_TRANSITION)
+		os_atomic_dec_long(&obs->data.active_transitions);
+
 	obs_context_data_remove(&source->context);
 
 	blog(LOG_INFO, "source '%s' destroyed", source->context.name);