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/proto/gui_gtk.pro b/src/proto/gui_gtk.pro
index 319e27d..7b87ba0 100644
--- a/src/proto/gui_gtk.pro
+++ b/src/proto/gui_gtk.pro
@@ -9,6 +9,8 @@
 void gui_mch_destroy_menu(vimmenu_T *menu);
 void gui_mch_set_scrollbar_thumb(scrollbar_T *sb, long val, long size, long max);
 void gui_mch_set_scrollbar_pos(scrollbar_T *sb, int x, int y, int w, int h);
+int gui_mch_get_scrollbar_xpadding(void);
+int gui_mch_get_scrollbar_ypadding(void);
 void gui_mch_create_scrollbar(scrollbar_T *sb, int orient);
 void gui_mch_destroy_scrollbar(scrollbar_T *sb);
 char_u *gui_mch_browse(int saving, char_u *title, char_u *dflt, char_u *ext, char_u *initdir, char_u *filter);