[pdftex] missing space chars at font switches

[Karl Berry]

    So at some point (last 1-2 years?) the algorithm must have changed,
    or some parameter has been given a different value.

I see nothing immediately obvious in the NEWS and ChangeLog.
But I don't doubt you that something must have changed.

    Can we please revisit this.

Thanh said he hopes to deal with this for TL25.
Patches would surely be welcome :). --thanks, karl.