Refactoring DrawLooper so that it can apply shadow effects as skia image filters

LayoutTests/fast/canvas/script-tests/canvas-draw-canvas-on-canvas-shadow.js

 description("Ensure correct behavior when drawing a canvas on a canvas with shadows. A blue and red checkered pattern should be displayed.");
 
 function print(message, color)
 {
     var paragraph = document.createElement("div");
     paragraph.appendChild(document.createTextNode(message));
     paragraph.style.fontFamily = "monospace";
     if (color)
         paragraph.style.color = color;
     document.getElementById("console").appendChild(paragraph);
 }
 
 function shouldBeAround(a, b)
 {
     var evalA;
     try {
         evalA = eval(a);
     } catch(e) {
         evalA = e;
     }
 
-    if (Math.abs(evalA - b) < 15)
+    if (Math.abs(evalA - b) < 20)
         print("PASS " + a + " is around " + b , "green")
     else
         print("FAIL " + a + " is not around " + b + " (actual: " + evalA + ")", "red");
 }
 
 var canvas = document.createElement('canvas');
 document.body.appendChild(canvas);
 canvas.setAttribute('width', '600');
 canvas.setAttribute('height', '600');
 var ctx = canvas.getContext('2d');
@@ skipping 91 matching lines @@
 shouldBe('d[0]', '255');
 shouldBe('d[1]', '0');
 shouldBe('d[2]', '0');
 shouldBeAround('d[3]', '50');
 
 d = ctx.getImageData(450, 500, 1, 1).data;
 shouldBe('d[0]', '255');
 shouldBe('d[1]', '0');
 shouldBe('d[2]', '0');
 shouldBeAround('d[3]', '50');