Current column in non-selected (inactive) window.

[Keith David Bershatsky]

I am working on feature requests #17684 (crosshairs) and #22873 (multiple fake 
cursors).  [Crosshairs changes colors depending upon whether the current column 
is odd/even numbered, or beyond the fill-column.]

I was unable to find a built-in way to obtain the current column of point in 
the non-selected (inactive) window.  I believe the problem is that 
current_column and its underling functions use PT and PT_BYTE, which is not the 
same as the window_point in the non-selected (inactive) window.

I resolved the issue by creating a new function (nearly identical to 
current_column and its underlying functions) that now take an argument for a 
particular point of interest, and I am using Fwindow_point to get that 
interesting point.  I replaced all PT with my particular point of interest, and 
in a couple of places, I replaced PT_BYTE with CHAR_TO_BYTE (point_of_interest).

Other than creating those new functions as described above, is there a built-in 
mechanism to get the current column in a non-selected (inactive) window?

If nothing exists, perhaps this new feature might be useful to other users in 
the future?