overlay: fps window clean up (#506)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Use fmt library for formatting time

Round down sub-second units so that the FPS counter only updates once
per second (instead of Time and Uptime ticking independently)

clean up usage of `localtime` in `logging.cpp`

src/spice2x/overlay/windows/fps.cpp

@@ -1,5 +1,6 @@
 #include <iomanip>
 #include <sstream>
+#include "external/fmt/include/fmt/chrono.h"
 #include "fps.h"
 
 namespace overlay::windows {
@@ -15,7 +16,8 @@ namespace overlay::windows {
                       | ImGuiWindowFlags_NoNavInputs
                       | ImGuiWindowFlags_NoDocking;
         this->bg_alpha = 0.5f;
-        this->start_time = std::chrono::system_clock::now();
+        this->start_time =
+            std::chrono::floor<std::chrono::seconds>(std::chrono::system_clock::now());
     }
 
     void FPS::calculate_initial_window() {
@@ -34,23 +36,23 @@ namespace overlay::windows {
         ImGui::Text("FPS:  %.1f", io.Framerate);
         // ImGui::Text("FT: %.2fms", 1000 / io.Framerate);
 
-        auto now = std::chrono::system_clock::now();
+        const auto now = std::chrono::system_clock::now();
+        const auto now_s = std::chrono::floor<std::chrono::seconds>(now);
 
         // current time
         {
-            auto now_t = std::chrono::system_clock::to_time_t(now);
+            const std::time_t tt = std::chrono::system_clock::to_time_t(now_s);
-            static CHAR buf[48];
+            std::tm local_tm{};
-            std::strftime(buf, sizeof(buf), "Time: %H:%M:%S", std::localtime(&now_t));
+            localtime_s(&local_tm, &tt);
-            ImGui::Text(buf);
+            ImGui::TextUnformatted(fmt::format("Time: {:%H:%M:%S}", local_tm).c_str());
         }
 
         // elapsed time
         {
-            auto d = now - this->start_time;
+            const auto uptime = now_s - this->start_time;
-            const auto h = std::chrono::duration_cast<std::chrono::hours>(d);
+            ImGui::TextUnformatted(
-            const auto m = std::chrono::duration_cast<std::chrono::minutes>(d - h);
+                fmt::format("Up:   {:%H:%M:%S}",
-            const auto s = std::chrono::duration_cast<std::chrono::seconds>(d - h - m);
+                    std::chrono::floor<std::chrono::seconds>(uptime)).c_str());
-            ImGui::Text("Up:   %02d:%02d:%02d", (int)h.count(), (int)m.count(), (int)s.count());
         }
     }
 }

src/spice2x/util/logging.cpp

@@ -13,8 +13,9 @@ std::string_view log_get_datetime() {
 std::string_view log_get_datetime(std::time_t now) {
     static thread_local char buf[64];
 
-    // `localtime` on Windows is thread-safe
+    std::tm local_tm{};
-    strftime(buf, sizeof(buf), "[%Y/%m/%d %X]", localtime(&now));
+    localtime_s(&local_tm, &now);
+    strftime(buf, sizeof(buf), "[%Y/%m/%d %X]", &local_tm);
 
     return buf;
 }