refactor FpsCounter index increment

GitOrigin-RevId: f8c636747e

Jumpy.Shared/FpsCounter.cs

@@ -17,10 +17,7 @@ namespace Jumpy {
         fps = 1000.0 * frameTimes.Length / timeElapsed;
       }
       frameTimes[idx] = now;
-      idx++;
-      if (idx == frameTimes.Length) {
-        idx = 0;
-      }
+      idx = (idx + 1) % frameTimes.Length;
     }
   }
 }