One way to think about text encoding is that it provides a nice way to represent indexes into a big array of glyphs: "unicode". Fonts map (some) of these indexes to graphic representations and provide "font hinting" -- clues as to how different letters should be rendered at different sizes -- and kerning information.

Usually your desktop environment is the thing that handles all of this, providing widgets for other programs to combine to create GUIs. I can't find it right now, but you can probably find e.g. GNOME/GTK's implementation.

(Re: Textmate: code is typically rendered in monospace font -- where every glyph has the same size -- so that it has the same alignment in every font with every renderer.)