Support `devicePixelRatio` !== 1. Fixes resize issues on Retina displays.

@allisoncorey @eric79 @BrettASwift, this keeps the canvas' CSS size correct on high-resolution displays that  have multiple physical pixels per CSS pixel. It also sizes the canvas internally for physical pixels to enable high-resolution rendering.

Heh, heh ... best explanation ever.  :wink:

So, with that said, should the calculation for `width` be:

    self.width = mycanvas.clientWidth * sceneNode.renderer.devicePixelRatio;

?