Finds a SignalSource given a pair of context and ID.
It is a programmer error to attempt to look up a non-existent source.
More specifically: source IDs can be reissued after a source has been destroyed and therefore it is never valid to use this function with a
source ID which may have already been removed. An example is when scheduling an idle to run in another thread with
idle may already have run and been removed by the time this function is called on its (now invalid) source ID. This source ID may have been
reissued, leading to the operation being performed against the wrong source.
a MainContext (if null, the default context will be used)
the source ID, as returned by get_id.