libobs: Add obs_source_info.filter_add

If there is filter_remove, it is reasonable to expect that there is also
filter_add. filter_add also enables filters to attach signal handlers
on the parent, and disconnect them in filter_remove.

docs/sphinx/reference-sources.rst

@@ -410,6 +410,15 @@ Source Definition Structure (obs_source_info)
    :param event:       Key event properties
    :param focus:       Key event type (true if mouse-up)
 
+.. member:: void (*obs_source_info.filter_add)(void *data, obs_source_t *source)
+
+   Called when the filter is added to a source.
+
+   (Optional)
+
+   :param  data:   Filter data
+   :param  source: Source that the filter is being added to
+
 .. member:: void (*obs_source_info.filter_remove)(void *data, obs_source_t *source)
 
    Called when the filter is removed from a source.
@@ -1547,7 +1556,7 @@ Filters
    reference.
 
    Only guaranteed to be valid inside of the video_render, filter_audio,
-   filter_video, and filter_remove callbacks.
+   filter_video, filter_add, and filter_remove callbacks.
 
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