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/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
-    strftime(buf, sizeof(buf), "[%Y/%m/%d %X]", localtime(&now));
+    std::tm local_tm{};
+    localtime_s(&local_tm, &now);
+    strftime(buf, sizeof(buf), "[%Y/%m/%d %X]", &local_tm);
 
     return buf;
 }