The cairo-dock program was passing a cairo_t in an error state to
rsvg_handle_render_cairo(), and so we failed deep in the innards of
librsvg when cairo-rs validates the cairo::Context's status.

Cairo-dock was doing something like

  surf = cairo_image_surface_create (... an invalid size ...);
  cr = cairo_create (surf);

  rsvg_handle_render_cairo (handle, cr);    // we now catch the error right here

We catch invalid cr's, emit a warning, and return FALSE from
rsvg_handle_render_cairo*().

#276