anchor.diff:  * bind image_registry between scratch and main document so we don't process the same image more than once
     - need to do some testing
   * show SVG warnings if debug (or trace) is on
  -* a nested image with a relative path in an SVG is resolved relative to the SVG source file (document in README)
   * clean temporary files once per conversion? (file issue)
   * clear font paths in SVG interface so it doesn't scan system? (since it's not portable anyway)
   * title is being rendered 3 times (maybe one for scratch?); explain why in comments if normal
  
positive.code: * check that color of table edges set to table_grid_color??
  * shouldn't the default align for headings be $base_align??
  * should not wrap at formatting mark unless it is at a break opportunity
  * be more consistent with how examples are shown in theming-guide.adoc (do we show last segment only?)
  * allow top as alternative to margin_top for all elements on title page (#431)
  * swallowing exceptions! (any use of e.message in a string is dangerous)
  
negative.code: * /PageLabels/Nums must have entry for every page in front matter, even if a blank page
    - in fact, must account for every page or else numbering lags behind when scrolling document
  * if we set the vposition on image to a numeric value, it skips the overrun check that happens internally
  * any instance variables referenced by converter methods for inline nodes could get accessed before the converter for document is called
  
  == Potential Optimizations
  
  