[cairo] Contributing with Underline and strikeout code

[Jordi Mas]

Hello!

This is Jordi. In my related work for Mono I had to implement font
underline and strikeout in fonts for libgdiplus. Currently is not available in 
Cairo.

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.

Thanks,

Jordi,