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/plugin/bridges/vst2.cpp

@@ -19,6 +19,14 @@
 #include "../../common/communication/vst2.h"
 #include "../utils.h"
 
+/**
+ * The interval in seconds between synchronizing the Wine plugin host's audio
+ * thread scheduling priority with the host's audio thread.
+ *
+ * @relates Vst2Bridge::last_audio_thread_priority_synchronization
+ */
+constexpr time_t audio_thread_priority_synchronization_interval = 10;
+
 intptr_t dispatch_proxy(AEffect*, int, int, intptr_t, void*, float);
 void process_proxy(AEffect*, float**, float**, int);
 void process_replacing_proxy(AEffect*, float**, float**, int);
@@ -497,7 +505,19 @@ void Vst2PluginBridge::do_process(T** inputs, T** outputs, int sample_frames) {
                     input_buffers[channel].begin());
     }
 
-    const AudioBuffers request{input_buffers, sample_frames};
+    // We'll synchronize the scheduling priority of the audio thread on the Wine
+    // plugin host with that of the host's audio thread every once in a while
+    std::optional<int> new_realtime_priority = std::nullopt;
+    time_t now = std::time(nullptr);
+    if (now > last_audio_thread_priority_synchronization +
+                  audio_thread_priority_synchronization_interval) {
+        new_realtime_priority = get_realtime_priority();
+        last_audio_thread_priority_synchronization = now;
+    }
+
+    const AudioBuffers request{.buffers = input_buffers,
+                               .sample_frames = sample_frames,
+                               .new_realtime_priority = new_realtime_priority};
     sockets.host_vst_process_replacing.send(request, process_buffer);
 
     // Write the results back to the `outputs` arrays