PNG Renderer: Font and Text Features

[shawn-mcgee]

Fix #74. Font loading still needs some rearranging but it works.
Fix #75. Shrink-to-fit is also working well enough, although we may want to investigate adding some degree of padding to the text (jsPDF did this by default). The current implementation leaves no gaps between text and borders.

[lorennorman]

Couple things we need:

• can PNG render all of the built-in PDF fonts+variants?
  • they are: helvetica, courier, times, symbol, zapfdingbats in normal, bold, italic, bolditalic
• can the installed system fonts be exposed to Paperize? whatever the browser already has access to
• if we go to render a font but it isn't available, how can we fail gracefully and also explain this to the user?
• side-by-side renders of PDF and PNG should be as close as possible, can we get them pixel perfect?