[Jack] port latency miscalculation

[jean-emmanuel]

The following setup

[loop 1 "pre-fx"] --> [jack client with non-null latency] --> [loop 2 "post-fx"]

Results in sooperlooper declaring a forever-increasing port latency and makes it impossible to use along with a latency-aware software such as ardour. I suspect it's related to the use of the deprecated jack_port_get_total_latency function to compute the port latency. Using 2 separate instances of sooperlooper for pre and post-fx tracks would break SL's tight loop synchronization.