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

CHANGELOG.md

@@ -78,7 +78,9 @@ TODO: Add an updated screenshot with some fancy VST3-only plugins to the readme
     to realtime priority. This prevents changing the scheduling policy of your
     host's GUI thread if your host instantiates plugins from its GUI thread like
     REAPER does.
-  - TODO: Next up is periodically synchronizing audio thread priorities.
+  - The realtime scheduling priorities of all audio threads on the Wine plugin
+    host are now periodically synchronized with those of the host's audio
+    threads.
 
 - Opening and closing plugin editors is now also no longer done with realtime
   priority. This should get rid of any latency spikes during those operations,