Improve thread safety for scene items

Scene items previously were removed by calling obs_sceneitem_destroy,
but this proved to be a potential race condition where two different
threads could try to destroy the same scene item at the same time.

Instead of doing that, reference counting is now used on scene items,
and an explicit obs_sceneitem_remove function is used instead for item
removal, which sets a 'removed' variable to ensure it can only be called
exactly one time.

libobs/obs.h

@@ -501,14 +501,16 @@ EXPORT void obs_scene_enum_items(obs_scene_t scene,
 /** Adds/creates a new scene item for a source */
 EXPORT obs_sceneitem_t obs_scene_add(obs_scene_t scene, obs_source_t source);
 
-/** Removes/destroys a scene item.  Returns the source reference counter
- * (if any) */
-EXPORT int obs_sceneitem_destroy(obs_sceneitem_t item);
+EXPORT int obs_sceneitem_addref(obs_sceneitem_t item);
+EXPORT int obs_sceneitem_release(obs_sceneitem_t item);
 
-/** Gets the scene parent associated with the scene item */
+/** Removes a scene item. */
+EXPORT void obs_sceneitem_remove(obs_sceneitem_t item);
+
+/** Gets the scene parent associated with the scene item. */
 EXPORT obs_scene_t obs_sceneitem_getscene(obs_sceneitem_t item);
 
-/** Gets the source of a scene item */
+/** Gets the source of a scene item. */
 EXPORT obs_source_t obs_sceneitem_getsource(obs_sceneitem_t item);
 
 /* Functions for gettings/setting specific oriantation of a scene item */