Window coordinates

For #91.

Modify `Transforms.pointToWindowCoordinates` and `SceneTransforms.wgs84ToWindowCoordinates` to return coordinates where the origin is at the top left.

You can test it by adding this code to the Picking Sandcastle example:
```javascript
        // Mouse over the globe to see the cartographic position
        handler = new Cesium.ScreenSpaceEventHandler(scene.canvas);
        handler.setInputAction(function(movement) {
            var cartesian = scene.camera.pickEllipsoid(movement.endPosition, ellipsoid);
            if (cartesian) {
                //var windowCoordinates = Cesium.SceneTransforms.wgs84ToWindowCoordinates(scene, cartesian);
                
                var context = scene._context;
                var viewProjMatrix = context.uniformState.viewProjection;
		        var viewport = new Cesium.BoundingRectangle();
		        viewport.width = context.drawingBufferWidth;
		        viewport.height = context.drawingBufferHeight;
		        var viewportTransformation = Cesium.Matrix4.computeViewportTransformation(viewport, 0.0, 1.0);
                var windowCoordinates = Cesium.Transforms.pointToWindowCoordinates(viewProjMatrix, viewportTransformation, cartesian);
                
                label.show = true;
                label.text = '(' + movement.endPosition.x + ', ' + Math.floor(windowCoordinates.x + 0.5) + '), (' + movement.endPosition.y + ', ' + Math.floor(windowCoordinates.y + 0.5) + ')';
                label.position = cartesian;
            } else {
                label.text = '';
            }
        }, Cesium.ScreenSpaceEventType.MOUSE_MOVE);
```
Each pair of numbers should be equal.