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.