bug#18493: 24.3.93; posn-col-row should take text-scale-mode into account

[Drew Adams <drew.adams@oracle.com>]

> > I don't even understand why the value should change with text
> > scale.
> 
> It would solve the problem of text-scale-mode being enabled in
> buffers, where I'm getting inaccurate results from `posn-col-row'
> because of that. And by "inaccurate", I mean different from the
> ones I'd like.

I don't understand why the value changing helps, instead of hurts,
in that context.  I would think that the column should not change
just because the text is scaled.  But I'm probably missing something.

> > What part of the description of `pos-col-row' would lead one to
> > think that this changes with text scale?
> 
> Probably none. Do you have code that calls `posn-col-row', though?

It doesn't matter whether I do or don't.  As a matter of fact, I do,
but only a little bit - getting the column of a mouse click, using:

 (car (posn-col-row (event-start event)))

And I guess that code must be broken wrt text scaling.  I didn't
realize that.

> Is it aware of `posn-col-row' not being affected by text scale?

I guess you mean *being* affected by it (?).  IIUC, the return
value depends on the text scale.  Isn't that the bug you reported?

That's the bug I see (though someone will no doubt explain why
they think it is TRT).

> Does it have explicit support for text scaling?

No, my code does not.  From what I understand now, I guess it
needs to worry about that now.  Seems nuts that it should have to,
but my understanding is limited...