[client] main: use the monotonic clock to calculate the ups/fps

The accumulated time is not the best way to do this as the timer
function callback may not be exactly every 1000ms, by using the
monotonic clock we will get more accurate results.

client/src/main.h

@@ -104,7 +104,6 @@ struct AppState
   RingBuffer            renderTimings;
   RingBuffer            frameTimings;
 
-  atomic_uint_least64_t renderTime;
   atomic_uint_least64_t renderCount, frameCount;
   _Atomic(float)        fps, ups;