Add Greyscale Function

lib/Dither.dart

@@ -1,6 +1,24 @@
 import 'dart:html';
 import 'dart:typed_data';
 
+class Pixed {
+  static PixelArray greyscale(PixelArray image) {
+    PixelArray pxArr = new PixelArray.fromPixelArray(image);
+
+    for (var y = 0; y < pxArr.getHeight(); y++) {
+      for (var x = 0; x < pxArr.getWidth(); x++) {
+        var px = pxArr.getPixel(x, y);
+        int average = ((px.r+px.g+px.b) / 3).round();
+        px.r = average;
+        px.g = average;
+        px.b = average;
+      }
+    }
+
+    return pxArr;
+  }
+}
+
 class Pixel {
   int x;
   int y;