Implement addPath() method for Path object

LayoutTests/fast/canvas/script-tests/canvas-path-addpath.js

+description("Test addPath() method.");
+var ctx = document.createElement('canvas').getContext('2d');
+
+debug("Test addPath() with transform as identity matrix.")
+ctx.beginPath();
+var p1 = new Path();
+p1.rect(0,0,100,100);
+var p2 = new Path();
+p2.rect(0,100,100,100);
+var svgElement = document.createElementNS("http://www.w3.org/2000/svg", "svg");
+var m = svgElement.createSVGMatrix();

[Justin Novosad, 2014/02/18 16:09:29]

simpler way to create an SVGMatrix: 
var m = ctx.currentTransform;

[pals, 2014/02/19 11:47:22]

Done.

+p1.addPath(p2, m);
+ctx.fillStyle = 'yellow';
+ctx.currentPath = p1;
+ctx.fill();
+var imageData = ctx.getImageData(0, 100, 100, 100);
+var imgdata = imageData.data;
+shouldBe("imgdata[4]", "255");
+shouldBe("imgdata[5]", "255");
+shouldBe("imgdata[6]", "0");
+shouldBe("imgdata[7]", "255");
+debug("");
+
+debug("Test addPath() with transform as translate(100, -100).")

[Justin Novosad, 2014/02/18 16:09:29]

Please add an additional test case where the transform is a non-invertible
matrix.

[pals, 2014/02/19 11:47:22]

Done.

+ctx.beginPath();
+var p1 = new Path();

[Justin Novosad, 2014/02/18 16:09:29]

re-declaration of existing variable.

[pals, 2014/02/19 11:47:22]

Done.

+p1.rect(0,0,100,100);
+var p2 = new Path();
+p2.rect(0,100,100,100);
+var svgElement = document.createElementNS("http://www.w3.org/2000/svg", "svg");

[Justin Novosad, 2014/02/18 16:09:29]

No need to re-create this

[pals, 2014/02/19 11:47:22]

Done.

+var m = svgElement.createSVGMatrix();
+m.a = 1; m.b = 0;
+m.c = 0; m.d = 1;
+m.e = 100; m.f = -100;
+p1.addPath(p2, m);
+ctx.fillStyle = 'yellow';
+ctx.currentPath = p1;
+ctx.fill();
+var imageData = ctx.getImageData(100, 0, 100, 100);
+var imgdata = imageData.data;
+shouldBe("imgdata[4]", "255");
+shouldBe("imgdata[5]", "255");
+shouldBe("imgdata[6]", "0");
+shouldBe("imgdata[7]", "255");
+debug("");
+
+debug("Test addPath() with transform as null.")
+var p1 = new Path();
+p1.rect(0,0,100,100);
+var p2 = new Path();
+shouldThrow("p1.addPath(p2, null)");

[Justin Novosad, 2014/02/18 16:09:29]

According to the spec, the transform is a nullable argument.  If transform is
null, we must just leaving the path untransformed (same as if transform was an
identity matrix).

[pals, 2014/02/19 11:47:22]

The addPath idl changes generates following code

static void addPathMethod(const v8::FunctionCallbackInfo<v8::Value>& info)
{
    if (UNLIKELY(info.Length() < 2)) {
        throwTypeError(ExceptionMessages::failedToExecute("addPath", "Path",
ExceptionMessages::notEnoughArguments(2, info.Length())), info.GetIsolate());
        return;
    }
    DOMPath* imp = V8Path::toNative(info.Holder());
    V8TRYCATCH_VOID(DOMPath*, path, V8Path::hasInstance(info[0],
info.GetIsolate()) ? V8Path::toNative(v8::Handle<v8::Object>::Cast(info[0])) :
0);
    V8TRYCATCH_VOID(RefPtr<SVGMatrixTearOff >, transform,
V8SVGMatrix::hasInstance(info[1], info.GetIsolate()) ?
V8SVGMatrix::toNative(v8::Handle<v8::Object>::Cast(info[1])) : 0);
    if (!transform) {
        throwTypeError(ExceptionMessages::failedToExecute("addPath", "Path",
"parameter 2 is not of type 'SVGMatrix'."), info.GetIsolate());
        return;
    }
    imp->addPath(path, transform->propertyReference());
}

So, we never get transform as NULL. 

+debug("");