1. on font initialization - creates font.missingGlyphs = string of all the available characters in the loaded font file
2. sanitize (to be valid regexp) and build font.missingGlyphsRegexp
3. on render process: if "part" (as defined in cufon call) has a missing char - then don't render it and pass it on normally.


This allows both non-rendered and renders fonts to be (unilke the original cufon which replaces non existing glyphs with $missingGlyph  (default blank))