add fps counter

Sources/Voxelotl/FPSCalculator.swift

@@ -0,0 +1,18 @@
+import Foundation
+
+public struct FPSCalculator {
+  private var _accumulator = 0.0
+  private var _framesCount = 0
+
+  public mutating func frame(deltaTime: Double, result: (_ fps: Int) -> Void) {
+    _framesCount += 1
+    _accumulator += deltaTime
+
+    if (_accumulator >= 1.0) {
+      result(_framesCount)
+
+      _framesCount = 0
+      _accumulator = fmod(_accumulator, 1.0)
+    }
+  }
+}