bug#18385: 24.3.93; posn-at-point doesn't account for tab-width

[Dmitry]

1. New, empty buffer.
2. (insert "a\tb")
3. (posn-actual-col-row (posn-at-point))
=> (3 . 0)

It should probably return (9 . 0).

I'm not 100% this is actually a bug, but (posn-actual-col-row
(posn-at-point)) returns the "visually" correct column values in the
more complex cases (after text with `display' or `compose-region' called
on it), so not accounting for tab-width looks surprising.

Originally: https://github.com/company-mode/company-mode/issues/175

In GNU Emacs 24.3.93.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-08-18 on axl
Repository revision: 117447 eliz@gnu.org-20140817144850-xgexz1n2z8s4aiur
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04.1 LTS