[matthiaskramm]

> but it describes a problem that hasn't been considered 'interesting' for decades

Actually, polygon operations on grids is still an active research area, with the latest papers on the topic less than two weeks old: http://www.sci.utah.edu/socg2010agenda.html

> Why would they need to run the same intersection logic over and over [...]?

We usually don't. It basically depends on the context in which the glyphs appear on the page. For a standard, say, LaTeX document consisting of mostly text and without weird graphic operations taking place around or on top of the text, a given glyph is just processed once.