bug#29002: it.first_visible_x is erroneously 0 while horizontal scrolling.

[Eli Zaretskii]

> Date:  Sun, 29 Oct 2017 10:29:29 -0700
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  29002@debbugs.gnu.org
> 
> Here is a link to screen-shot #3:
> 
> https://www.lawlist.com/images/bug_hscl_03.png
> 
> In that screen-shot, we make the following observations:
> 
> * A total of 5 screen lines.
> 
> * Line 1 is the current line.
> 
> * All 5 lines visually appear to be horizontally scrolled.
> 
> * Using the revised function bug-hscroll (below), we iterate over each screen 
> line and generate the values for it.first_visible_x and it.vpos.
> 
> * it.first_visible_x returns as being 0 for each of the 5 screen lines.

This cannot be true: if every line in the window is hscrolled,
first_visible_x must be non-zero, right after the call to
start_display.  If first_visible_x is zero, only the current line can
be hscrolled.