bug#32975: Emacs 26.1; GTK Warnings

[martin rudalics]

> There is only one call to xg_set_background_color, from xfns.c, which looks
> like this with my patch applied:
>
> #ifdef USE_TOOLKIT_SCROLL_BARS
> # ifdef USE_GTK
>        xg_set_background_color (f, bg);
> # endif
> #else
>
> So it will only ever happen when the scrollbars are GTK widgets.

But this effectively throws out the baby - painting the background of
the frame widget proper via

      xg_set_widget_bg (f, FRAME_GTK_WIDGET (f), FRAME_BACKGROUND_PIXEL (f));

with the bathwater - painting the background of non-existent scroll
bar widgets via

          xg_set_widget_bg (f, webox, FRAME_BACKGROUND_PIXEL (f));

for GTK builds with native Emacs scroll bars.  Maybe on X the

      XSetWindowBackground (dpy, FRAME_X_WINDOW (f), bg);

in x_set_background_color would take care of that.  But are we sure?

martin