Hi Eli and Stephen,

I tried to understand the different librsvg APIs (which is quite a mess)
and AFAIU, rsvg_handle_get_geometry_for_layer use a viewport as input to
render the SVG.  See, here:
https://gnome.pages.gitlab.gnome.org/librsvg/Rsvg-2.0/method.Handle.get_geometry_for_layer.html

So in the case we have already read a viewbox, I'm using it as the
viewport rectangle.  I don't know if it is the best way but at least it
works for me (i.e the image does not disappear below the baseline)