Returns the numeric ID for a particular source.
The ID of a source is a positive integer which is unique within a particular main loop context. The reverse mapping from ID to source is done by find_source_by_id.
You can only call this function while the source is associated to a MainContext instance; calling this function before attach or after destroy yields undefined behavior. The ID returned is unique within the MainContext instance passed to attach.
the ID (greater than 0) for the source