[client] main: improve frame rate sync lock with the guest

If the guest is not sending frames at a constant rate, the minimum FPS
timeout may expire drawing an additional frame. This change calculates
the average ups frame time over the past 100ms and adds this to the
timeout value allowing this value to be dynamic.

client/src/main.h

@@ -104,7 +104,7 @@ struct AppState
   RingBuffer            renderTimings;
   RingBuffer            frameTimings;
 
-  atomic_uint_least64_t renderCount, frameCount;
+  atomic_uint_least64_t renderCount, frameCount, upsTime;
   _Atomic(float)        fps, ups;
 
   uint64_t resizeTimeout;