Do not rely on scene measurements during retexture.

The `DSMultilineLabelNode` might not yet have been added to an `SKScene`.

Instead, default `paragraphWidth` using scene if it exists, or `UIScreen`/`NSScreen` width otherwise.
When calculating bounding box for text, use `MAXFLOAT` for height so that it is unrestricted.

(I'm happy to use different defaults if that sounds good to you.  The only thing I care about is either avoiding relying on the scene, or else retexturing again once the node is added to the scene.)