patch 8.2.2939: GTK: righthand scrollbar does not show with split window

Problem:    GTK: righthand scrollbar does not show with split window.
Solution:   Adjust padding when two scrollbars are used. (Matt Wozniski,
            closes #8324)
diff --git a/src/gui_gtk.c b/src/gui_gtk.c
index 77076fc..79a1ac3 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -1050,6 +1050,9 @@
     xpad = gui.formwin->allocation.width - gui.drawarea->allocation.width
 							 - gui.scrollbar_width;
 #endif
+    if (gui.which_scrollbars[SBAR_LEFT] && gui.which_scrollbars[SBAR_RIGHT])
+	xpad -= gui.scrollbar_width;
+
     return (xpad < 0) ? 0 : xpad;
 }