Library has scale factor; can be inferred from asset

XflLibrary can now have a scale factor assigned to it that will be multiplied with baseScale before preview/export.

Artists can embed scale information for an asset within the .fla by exporting a symbol named 'scale', and setting the desired scale as scaleX and scaleY on a symbol in layer 0, keyframe 0.  The symbol 'scale' is used for this metadata only, and will not be exported.