add forceSmartPSDPixelScaling to getPixmap

In PS <= 15.0, PSD smart objects were always scaled in pixel space, which led to fuzzy vectors.

In PS >= 15.1, PSD smart objects will now be scaled in a vector-preserving way. This pull request adds an option to `getPixmap` to force the old behavior.

Also, this opportunistically organizes some of the other transform options in `getLayerPixmap.jsx`