patch 8.2.3524: GUI: ligatures are not used

Problem:    GUI: ligatures are not used.
Solution:   Add the 'guiligatures' option. (Dusan Popovic, closes #8933)
diff --git a/src/proto/gui.pro b/src/proto/gui.pro
index d762420..209e7f1 100644
--- a/src/proto/gui.pro
+++ b/src/proto/gui.pro
@@ -7,6 +7,7 @@
 void gui_shell_closed(void);
 int gui_init_font(char_u *font_list, int fontset);
 int gui_get_wide_font(void);
+void gui_set_ligatures(void);
 void gui_update_cursor(int force, int clear_selection);
 void gui_position_menu(void);
 int gui_get_base_width(void);
diff --git a/src/proto/gui_gtk_x11.pro b/src/proto/gui_gtk_x11.pro
index 1d0a78b..3fa2ac9 100644
--- a/src/proto/gui_gtk_x11.pro
+++ b/src/proto/gui_gtk_x11.pro
@@ -43,6 +43,7 @@
 void gui_mch_set_bg_color(guicolor_T color);
 void gui_mch_set_sp_color(guicolor_T color);
 int gui_gtk2_draw_string(int row, int col, char_u *s, int len, int flags);
+int gui_gtk2_draw_string_ext(int row, int col, char_u *s, int len, int flags, int force_pango);
 int gui_mch_haskey(char_u *name);
 int gui_get_x11_windis(Window *win, Display **dis);
 Display *gui_mch_get_display(void);