Sync VST2 audio thread scheduling priorities

We'll periodically copy the scheduling priorities from the host's audio
threads to the Wine plugin host's audio threads. The overhead of doing
this is about 1 microsecond on my system, so doing this every cycle
really adds up. But getting the Unix epoch time and comparing some
timestamps has a neglegible overhead, so this should give you the best
of both worlds.

Next we'll do the same thing for VST3 plugins.

As suggested by @jhernberg

src/common/utils.cpp

@@ -31,7 +31,7 @@ fs::path get_temporary_directory() {
     }
 }
 
-std::optional<int> get_scheduling_priority() {
+std::optional<int> get_realtime_priority() {
     sched_param current_params{};
     if (sched_getparam(0, &current_params) == 0 &&
         current_params.sched_priority > 0) {