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/serialization/vst2.h

@@ -555,6 +555,15 @@ struct AudioBuffers {
      */
     int sample_frames;
 
+    /**
+     * We'll periodically synchronize the realtime priority setting of the
+     * host's audio thread with the Wine plugin host. We'll do this
+     * approximately every ten seconds, as doing this getting and setting
+     * scheduler information has a non trivial amount of overhead (even if it's
+     * only a single microsoecond).
+     */
+    std::optional<int> new_realtime_priority;
+
     template <typename S>
     void serialize(S& s) {
         s.ext(
@@ -572,5 +581,8 @@ struct AudioBuffers {
                 },
             });
         s.value4b(sample_frames);
+
+        s.ext(new_realtime_priority, bitsery::ext::StdOptional{},
+              [](S& s, int& priority) { s.value4b(priority); });
     }
 };