This is mainly relevant for Mixbus, which allows cyclic-connections
to record Master-out on a Track (which unconditionally has Mixbus send
and feeds master).
In Ardour it may also cause issues when creating loopback connections,
however latency is not usually updated with invalid graphs (old
process graph remains in use).
Otherwise it fixes a crash connecting Track 1 -> Track 2 -> Track 1.
This also optimizes Route::output_effectively_connected by caching
any prior lookup. This helps e.g. Track 1 -> Track 2 -> Master.
The connection "Track 2 -> Master" now only need to be looked up once.
See also 7958031287 and a556e96ed0
Robin Gareus
95693f9364
