libobs: Change return type for getting filter index

This changes the return type for getting the filter index from a
size_t to an int. This makes it easier for developers to use,
as an invalid index just returns a -1.

UI/window-basic-filters.cpp

@@ -1281,7 +1281,7 @@ void OBSBasicFilters::FiltersMoved(const QModelIndex &, int srcIdxStart, int,
 		neighborIdx = 0;
 
 	OBSSource neighbor = GetFilter(neighborIdx, isAsync);
-	size_t idx = obs_source_filter_get_index(source, neighbor);
+	int idx = obs_source_filter_get_index(source, neighbor);
 
 	OBSSource filter = GetFilter(list->currentRow(), isAsync);
 	obs_source_filter_set_index(source, filter, idx);

libobs/obs-source.c

@@ -3294,20 +3294,20 @@ void obs_source_filter_set_order(obs_source_t *source, obs_source_t *filter,
 		obs_source_dosignal(source, NULL, "reorder_filters");
 }
 
-size_t obs_source_filter_get_index(obs_source_t *source, obs_source_t *filter)
+int obs_source_filter_get_index(obs_source_t *source, obs_source_t *filter)
 {
 	if (!obs_source_valid(source, "obs_source_filter_get_index"))
-		return DARRAY_INVALID;
+		return -1;
 	if (!obs_ptr_valid(filter, "obs_source_filter_get_index"))
-		return DARRAY_INVALID;
+		return -1;
 
 	size_t idx;
 
 	pthread_mutex_lock(&source->filter_mutex);
 	idx = da_find(source->filters, &filter, 0);
 	pthread_mutex_unlock(&source->filter_mutex);
 
-	return idx;
+	return idx != DARRAY_INVALID ? (int)idx : -1;
 }
 
 static bool set_filter_index(obs_source_t *source, obs_source_t *filter,

libobs/obs.h

@@ -1134,8 +1134,8 @@ EXPORT void obs_source_filter_set_order(obs_source_t *source,
 					enum obs_order_movement movement);
 
 /** Gets filter index */
-EXPORT size_t obs_source_filter_get_index(obs_source_t *source,
-					  obs_source_t *filter);
+EXPORT int obs_source_filter_get_index(obs_source_t *source,
+				       obs_source_t *filter);
 
 /** Sets filter index */
 EXPORT void obs_source_filter_set_index(obs_source_t *source,