bug#20619: Scrollbars

[Eugen Dedu]

So I have successfully compiled emacs 25.1.1 to make tests.

I applied the patch given in message #13, but it works partially, I will 
discuss about it later.

Now, to advance emacs support for HIDPI I would like to fix the 
scrollbar.  Do all people here agree that the scrollbar has a width 
twice as normal?  The reason is that in src/gtkutil.c there is this code:

int
xg_get_default_scrollbar_width (void)
{
  return scroll_bar_width_for_theme * xg_get_gdk_scale ();
}

where xg_get_gdk_scale returns GDK_SCALE variable, i.e. 2 in general.

If I replace with:
  return scroll_bar_width_for_theme;
the scrollbar is shown correctly.

This change was made by 
https://github.com/emacs-mirror/emacs/commit/c0055ff5b03c9121ab5bf752496b09416f0f0a7d. 
 I think there was an error there, or perhaps in the mean time (since 
May 2015) GTK has changed in a way so that scrollbars are taken into 
account.

Anyway, using "GTK_SCALE=2 emacs" shows correctly the scrollbar with my 
proposition.

Note that GDK_DPI_SCALE is only for font, AFAIU from 
https://developer.gnome.org/gtk3/stable/gtk-x11.html.

What do you think?  Would you commit such a modification?

I would like to look into other issues as well.