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

LayoutTests/fast/canvas/script-tests/canvas-drawImage-shadow.js

 description("Ensure correct behavior of canvas with image shadow. A square with a cut-out top-right corner should be displayed with solid shadow (top) and blur shadow (bottom).");
 
 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);
@@ skipping 115 matching lines @@
     shouldBe('d[2]', '255');
     shouldNotBe('d[3]', '255');
 
     imageData = ctx.getImageData(300, 500, 1, 1);
     d = imageData.data;
     shouldBe('d[0]', '0');
     shouldBe('d[1]', '0');
     shouldBe('d[2]', '255');
     shouldNotBe('d[3]', '255');
 
+    imageData = ctx.getImageData(476, 324, 1, 1);
+    d = imageData.data;
+    shouldBe('d[0]', '0');
+    shouldBe('d[1]', '0');
+    shouldBe('d[2]', '255');
+    shouldNotBe('d[3]', '255');
+
     imageData = ctx.getImageData(400, 500, 1, 1);
     d = imageData.data;
     shouldBe('d[0]', '0');
     shouldBe('d[1]', '0');
     shouldBe('d[2]', '255');
     shouldNotBe('d[3]', '255');
 
     imageData = ctx.getImageData(400, 400, 1, 1);
     d = imageData.data;
     shouldBe('d[0]', '0');
@@ skipping 11 matching lines @@
     d = imageData.data;
     shouldBe('d[0]', '0');
     shouldBe('d[1]', '0');
     shouldBe('d[2]', '0');
     shouldBe('d[3]', '0');
 
     finishJSTest();
 }
 
 window.jsTestIsAsync = true;