Allow a user to view the OSM political layer as well as the selected layer

[phargogh]

From @lmandle:

For Voila, would it be possible to have the map show the geographic boundaries while you define your AOI? I see that underneath the preview layer. It would be nice to have the option of showing it by itself to guide clipping -- for example I wanted to clip the STRM layer for Colombia and had to guess at the boundaries when drawing the AOI. I realize it could be tricky, b/c the political boundaries are not what we'd clip.