[cairo] Contributing with Underline and strikeout code

[Carl Worth]

On Mar 9, Jordi Mas wrote:
 > If you think that is interesting for Cairo, I can prepare a patch to add
 > underline font in Cairo and send it you. It wont be more than a few
 > lines of code and two new defines for it.

Jordi,

Thanks for the kind offer.

I know it wouldn't be much new code, but I do start to worry about
feature creep here. It can be hard to know where to draw the line, but
there should be a notion of "primitives" in cairo, where features that
are easily described as combinations of primitives belong
outside. Underline support probably falls into that camp.

Another issue is that providing all possible variations for
underlining might require a more complex API than generally exists
within cairo.

Finally, it's not obvious to me what the correct result would be for
cairo_show_glyphs when in underline mode.

-Carl