Shaves another 3% for DejaVu Sans long Latin strings.

Shaves a good 10% off DejaVu Sans with simple Latin text for me.
Now, DejaVu is very ChainContext-intensive, but it's also a very
popular font!

Makes more sense, optimization-wise.

I know...

I see about 8% speedup with long strings with DejaVu Sans.

This is apparently what Uniscribe does.  Test case is:

  SEEN FATHA TEH ALEF

with Arabic Typesetting.  Originally reported by Khaled Hosny.

To be used for correct mark attachment to first component of a
MultipleSubst output.  That's what Uniscribe does.