Horizontal centering in the display engine

[sbaugh@catern.com]

The display engine supports display text left-aligned and right-aligned.
far as I can tell, it doesn't support displaying text centered.  This
would be useful for several things:

- User interfaces
- Presentations (this is my use case)

Currently there are a number of modes which implement a pseudo-centering
by inserting spaces based on the size of the window.  This has several
issues, among them that it doesn't automatically recenter when the
window size changes.

So it would be nice if the display engine natively supported displaying
centered text.

How difficult would this be to implement?  Could someone (namely, me)
with no prior exposure to the notoriously-complex display engine code
implement it in, say, a week of hacking?

Thanks!