[client] audio: add latency tuning parameter

This adds a new `audio:bufferLatency` option which allows the user to
adjust the amount of buffering LG does over the absolute bare minimum. By
default, this is set large enough to absorb typical timing jitter from
Spice. Users may reduce this if they care more about latency than audio
quality.

client/src/main.h

@@ -201,6 +201,7 @@ struct AppParams
   bool              showCursorDot;
 
   int               audioPeriodSize;
+  int               audioBufferLatency;
 };
 
 struct CBRequest