Sets the callback function storing the data as a refcounted callback "object".
This is used internally. Note that calling set_callback_indirect assumes an initial reference count on
callback_data, and thus
callback_funcs->unref will eventually be called once more than
It is safe to call this function multiple times on a source which has already been attached to a context. The changes will take effect for the next time the source is dispatched after this call returns.
pointer to callback data "object"
functions for reference counting