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: * /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
  
  == Potential Optimizations
  
  
negative.code: * allow dynamic background image with page number in path
  * running header/footer covers content (perhaps just a limitation that needs to be documented)
  * document that palette-based transparent in PNGs is not supported
  * add broader character range to monospace font so we can drop fallback font by default (#282)
    - consider having a fallback for prose and fallback for literal
  * file issue for prawn to preserve space (even w/ guards, spaces don't preserve over wrapped lines)
  