System: Don't use exact sleep on core thread

Only need an exact sleep for pre-frame sleep, since the frame is scheduled
to finish just in time.

Otherwise waking slightly late is harmless, the video thread handles the
presentation timing.

In the worst case, input would get polled up to 0.5ms later than originally
expected which isn't going to hurt anything. If you have that little frame
budget, disable optimal frame pacing.

src/core/system.cpp

@@ -2348,9 +2348,17 @@ void System::Throttle(Timer::Value current_time, Timer::Value sleep_until)
   // If we are running the GDB server and have clients, then use it to sleep instead.
   // That way in a query->response->query->response chain, we don't process only one message per frame.
   if (GDBServer::HasAnyClients())
+  {
     GDBServer::PollUntil(sleep_until);
+  }
   else
-    Timer::SleepUntil(sleep_until, g_settings.display_optimal_frame_pacing);
+  {
+    // Only need an exact sleep for pre-frame sleep, since the frame is scheduled to finish just in time.
+    // Otherwise waking slightly late is harmless, the video thread handles the presentation timing.
+    // In the worst case, input would get polled up to 0.5ms later than originally expected which isn't
+    // going to hurt anything. If you have that little frame budget, disable optimal frame pacing.
+    Timer::SleepUntil(sleep_until, s_state.pre_frame_sleep);
+  }
 
 #if 0
   const Timer::Value time_after_sleep = Timer::GetCurrentValue();