As an experiment to remove this recursion, this commit does a few
things:

* Makes get_node_geometry() not call get_dimensions().  This was a
  semi-recursive call when the toplevel SVG has
  width/height="100%" (or any other percentage-based size), and no
  viewBox.  Instead, start with a 1x1 viewport that matches the
  temporary surface used to run the rendering/measuring loop.

* Make the case where an SVG has no dimensions (no width/height, no
  viewBox, no objects at all) *not* a rendering error.  Remove
  RenderingError::SvgHasNoSize and just render nothing.

This also makes tests/render-crash not check the result of
rsvg_handle_render_cairo().

We are just checking whether the rendering doesn't crash; not that
files actually render without errors.

For the same reason, don't even try to figure out the SVG's
dimensions; just render to a constant-sized surface.