patch 8.2.1228: scrollbars not flush against the window edges when maximised

Problem:    Scrollbars not flush against the window edges when maximised.
Solution:   Add padding. (Ken Takata, closes #5602, closes #6466)
diff --git a/src/gui.c b/src/gui.c
index dc408bb..4981d73 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -1418,11 +1418,13 @@
     if (gui.which_scrollbars[SBAR_BOTTOM])
 	gui_mch_set_scrollbar_pos(&gui.bottom_sbar,
 				  text_area_x,
-				  text_area_y + text_area_height,
+				  text_area_y + text_area_height
+					+ gui_mch_get_scrollbar_ypadding(),
 				  text_area_width,
 				  gui.scrollbar_height);
     gui.left_sbar_x = 0;
-    gui.right_sbar_x = text_area_x + text_area_width;
+    gui.right_sbar_x = text_area_x + text_area_width
+					+ gui_mch_get_scrollbar_xpadding();
 
     --hold_gui_events;
 }