[bug #60587] Allow font files to specify kern pairs for characters in different fonts

[Dave]

Follow-up Comment #4, bug #60587 (project groff):

Hi Branden,

You're talking about the data, whereas the (intended) scope of this bug is
merely the mechanism to read the data--of which none yet exists.

You're right that it's theoretically possible this data will grow out of
control once a mechanism exists for reading it.  But given the maturity of
software typesetting and the complete lack of this data at present, this seems
a remote concern.  Typographers don't seem to be champing at the bit to
generate cross-font kerning metrics.

I agree that it's reasonable to expect the user to define her own kern pairs
for unlikely and one-off combinations.  But there are common cases, like the
canonical example of an italic f followed by a roman right parenthesis, that
invariably require adjustment.  It shouldn't be up to every user to reinvent
this kerning wheel.  Relatively common glyph combinations should Just Work.

In short, my ambitions for this feature are much more modest than what you
fear.

If we did have an Heirloom-style cross-font .kernpair request (bug #44244),
for groff's default fonts we could handle such common combinations with a
simple startup tmac file of .kernpair requests, as I alluded in comment #2. 
But if efficiency is one of your concerns, this method seems far less
efficient: the tmac file would presumably cover all the font families that
ship with groff, but the user may be using only a small number of these.  On
the efficiency front, it's better to read and store this data only when it's
needed.

In any case, I absolutely agree that as a first step, it makes sense to
implement a cross-font user-invokable kerning request.  Right now, the
capability doesn't even exist in groff, so let's get over that hurdle.  Give
the user a tool to solve the problem first; then we can revisit the idea of
automating some of that work for the user.

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