Also fix missing metadata on Sholstim and Citra

Inkscape now exits before it finishes writing its files, somehow.
So, we need to wait for the files to be created. The Blender
add-on also now correctly configures the lineart to use the
secondary camera, which is rotated to avoid errors when rendering
orthographically.

Also update the add-on to allow for configuration of
potrace's alpha settings, which controls how strongly
it favors straight lines. This makes things like buildings
look a lot nicer.

Also, modify the Blender add-on so that mass can be
computed via density even if a volume is not being included