Fix Rendering when no Changes occured

Rendering checks a dirty flag which gets set whenever a cell updates by changing their state.

lib/src/Cell.dart

@@ -6,11 +6,16 @@ class Cell {
   List<Rule> surviveRules = new List<Rule>();
   List<Rule> birthRules = new List<Rule>();
 
+  /// For determining if render updates are necessary in [Grid].render() function
+  bool dirty = false;
+
   Cell([bool state = false]) : this.state = state;
 
   void advanceState() {
     this.state = this.nextState;
     this.nextState = false;
+
+    this.dirty = true;
   }
 
   void update(int neighbors) {