libobs: Deprecate obs_scene_sceneitem_from_source

obs_scene_sceneitem_from_source is problematic because there can be
multiple items of the same source in a scene, which the function doesn't
account for. In such a case, it would return the first item it finds,
which often might not be what a developer expects.

It was originally added for the undo/redo-operation of "Add New Source"
where the UI guarantees that the item is unique, but for a general case
it's not suitable.

docs/sphinx/reference-scenes.rst

@@ -326,6 +326,11 @@ Scene Item Functions
 
    :return: The sceneitem associated with a source in a scene. Returns NULL if not found.
 
+   .. deprecated:: 31.0
+      This function is problematic because there can be multiple items of the same source in a scene.
+      In that case, which of those this function will return is undefined.
+      If this is the behavior you need, manually use :c:func:`obs_scene_enum_items` instead.
+
 ---------------------
 
 .. function:: void obs_sceneitem_set_id(obs_sceneitem_t *item);

libobs/obs.h

@@ -1806,8 +1806,8 @@ EXPORT void obs_sceneitem_save(obs_sceneitem_t *item, obs_data_array_t *arr);
 EXPORT void obs_sceneitem_set_id(obs_sceneitem_t *sceneitem, int64_t id);
 
 /** Tries to find the sceneitem of the source in a given scene. Returns NULL if not found */
-EXPORT obs_sceneitem_t *obs_scene_sceneitem_from_source(obs_scene_t *scene,
-							obs_source_t *source);
+OBS_DEPRECATED EXPORT obs_sceneitem_t *
+obs_scene_sceneitem_from_source(obs_scene_t *scene, obs_source_t *source);
 
 /** Save all the transform states for a current scene's sceneitems */
 EXPORT obs_data_t *obs_scene_save_transform_states(obs_scene_t *scene,