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.
Sorry to hold up the PR, but why not do this once when writing the vertex attribute instead of doing it over and over again in the shader?  It's probably also a cleaner separation of HTML windows coordinates and WebGL window coordinates.