Fixes to the framelimiter to make it more accurate.

This change improves the accuracy of the framelimiter so it yields the expected result. Before this change, it was impossible to actually achieve the default maximum framerate of 120 FPS.

Please note that the Windows code does not have a sleep function with a high enough granularity to take advantage of the improved precision, so Windows builds will still have an imprecise FPS cap.

common/framelimit.cpp

@@ -48,12 +48,13 @@ void Frame_Limiter(FrameLimitFlags flags)
 #else
         auto frame_end = std::chrono::steady_clock::now();
 #endif
-        int64_t _ms_per_tick = 1000 / Settings.Video.FrameLimit;
-        auto remaining =
-            _ms_per_tick - std::chrono::duration_cast<std::chrono::milliseconds>(frame_end - frame_start).count();
-        if (remaining > 0) {
-            ms_sleep(unsigned(remaining));
+        unsigned int min_frame_time = 1000000 / Settings.Video.FrameLimit;
+        auto cur_frame_time = std::chrono::duration_cast<std::chrono::microseconds>(frame_end - frame_start).count();
+        if (cur_frame_time < min_frame_time) {
+            frame_start += std::chrono::microseconds{min_frame_time};
+            us_sleep(min_frame_time - cur_frame_time);
+        } else {
+            frame_start = frame_end;
         }
-        frame_start = std::chrono::steady_clock::now();
     }
 }

common/mssleep.h

@@ -16,6 +16,21 @@
 #include <time.h>
 #endif
 
+/**
+ * Yield the current thread for at least us microseconds.
+ */
+static inline void us_sleep(unsigned us)
+{
+#ifdef _WIN32
+    Sleep((us + 999) / 1000);
+#else
+    struct timespec ts;
+    ts.tv_sec = us / 1000000;
+    ts.tv_nsec = (us % 1000000) * 1000;
+    nanosleep(&ts, NULL);
+#endif
+}
+
 /**
  * Yield the current thread for at least ms milliseconds.
  */