This is a simple polygon font that renders a blocky 3x5 font. This is
currently implemented as a single global function that uses private
data to mask everything else. It could be an object if/when needed.

Astute code viewers might notice that this appears to be stolen from
the Rx code in the devember-2015 project, but in fact THAT project
stole it from here, where it was originally implemented.

Fortunately the work of making it TypeScript compatible was done at
that point, so I didn't have to do it here.