[gnome-shell] HACKING.md: Update sample code to use paint context

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 8c4d07ba92e3293e4b406c0cef20be8ddd77c9ce
Author: Jonas Ådahl <jadahl gmail com>
Date:   Thu Nov 28 11:19:27 2019 +0100

    HACKING.md: Update sample code to use paint context
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/827

 HACKING.md | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)
---
diff --git a/HACKING.md b/HACKING.md
index 05176c45f9..d1e23e03fa 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -163,11 +163,17 @@ you to inherit from a type to use it, you can do so:
              return [100, 100];
         }
 
-        vfunc_paint() {
+        vfunc_paint(paintContext) {
+             let framebuffer = paintContext.get_framebuffer();
+             let coglContext = framebuffer.get_context();
              let alloc = this.get_allocation_box();
-             Cogl.set_source_color4ub(255, 0, 0, 255);
-             Cogl.rectangle(alloc.x1, alloc.y1,
-                            alloc.x2, alloc.y2);
+
+             let pipeline = new Cogl.Pipeline(coglContext);
+             pipeline.set_color4ub(255, 0, 0, 255);
+
+             framebuffer.draw_rectangle(pipeline,
+                alloc.x1, alloc.y1,
+                alloc.x2, alloc.y2);
         }
     });
 ```