Finds a source with the given source functions and user data. If multiple sources exist with the same source function and user data, the first one found will be returned.
the source_funcs passed to Source.new.
the user data from the callback.
the source, if one was found, otherwise NULL
Finds a source with the given source functions and user data. If multiple sources exist with the same source function and user data, the first one found will be returned.