[gtk/gtk-3-24: 1/2] force redraw in glareawhen buffer_age is 0

[Emmanuele Bassi <ebassi src gnome org>]

commit 69334021a22a79f5fddb30f0872fcf3fc6bc1cf0
Author: Rene Hopf <renehopf mac com>
Date:   Sun May 10 21:21:08 2020 +0200

    force redraw in glareawhen buffer_age is 0
    
    Reading form the back buffer is not allowed on software renderers,
    and this is reported by the buffer age, so reading from GL_BACK
    should not be done when the age is 0
    
    Closes #64

 gdk/x11/gdkglcontext-x11.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/gdk/x11/gdkglcontext-x11.c b/gdk/x11/gdkglcontext-x11.c
index 821ef965d0..0d0422efa0 100644
--- a/gdk/x11/gdkglcontext-x11.c
+++ b/gdk/x11/gdkglcontext-x11.c
@@ -149,7 +149,7 @@ gdk_x11_window_invalidate_for_new_frame (GdkWindow      *window,
 
 
   invalidate_all = FALSE;
-  if (buffer_age == 0 || buffer_age >= 4)
+  if (buffer_age >= 4)
     {
       cairo_rectangle_int_t whole_window = { 0, 0, gdk_window_get_width (window), gdk_window_get_height 
(window) };
 
@@ -163,6 +163,10 @@ gdk_x11_window_invalidate_for_new_frame (GdkWindow      *window,
     }
   else
     {
+      if (buffer_age == 0)
+        {
+          invalidate_all = TRUE;
+        }
       if (buffer_age >= 2)
         {
           if (window->old_updated_area[0])