Initial support for BlendModes in HTML5 Canvas

Using [CanvasRenderingContext2D.globalCompositeOperation](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D.globalCompositeOperation).
For now, only for **Bitmap.blendMode** and the **blendMode** parameter in BitmapData.draw().
Also, some blending modes are not supported.