Re: init_iterator takes window

[Thien-Thi Nguyen]

   appearance can also change between two windows even
   in the same frame.

so there is a semantic gap between current `current-column' and one that
would take into account variable-width fonts, implying `current-column'
callers need to bifurcate their usage to call some "window-current-column"
if they care about variable-width fonts, and `current-column' otherwise.

alternatively, we can add an "&optional window" arg to `current-column',
which if a window would mean take variable-width fonts into account (using
that window), if t would mean look for some default window and use that,
and otherwise fall back to using the existing computation methods, w/o
taking any font info into account.

depending on how we support `(current-column t)', two callers may get
different return values.  it might be a good idea to delay this support
until after we vet the callers, to see what idioms are most useful to
abstract.

to rms: could you define precisely what is meant by "upward compatibility"
wrt `current-column' *usage*?  to fulfill that goal i need to understand
the concept fully, including from caller perspective.

thi