bug#32975: Emacs 26.1; GTK Warnings

[martin rudalics]

> We do, but I only see xg_set_widget_bg getting called on scrollbar
> widgets when USE_TOOLKIT_SCROLL_BARS is set, in which case they're GTK
> widgets, so thatʼs the right thing to do.

I apologize for my stubbornness.

xg_set_background_color (struct frame *f, unsigned long bg)

has

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

which IIUC does not work on scrollbar widgets.

martin