When the node is driving, we need to call trigger_process()
otherwise we don't and we simply need to wait for the process() to
be called, triggered by the driver node.
See #1484
SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).