resolves #651 take fallback font into account when calculating width of string (PR #788)
resolves #651 take fallback font into account when calculating width of string (PR #788)

- introduce rendered_width_of_string and rendered_width_of_char methods
- take width of char from fallback font into account if not glyph not present in font
- calculate width of conum using correct font
asciidoctor/asciidoctor-pdf,Hextremist/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,Hextremist/asciidoctor-pdf,dsisnero/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,dsisnero/asciidoctor-pdf