HACKING.md: Update sample code to use paint context

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/827

HACKING.md

@@ -163,10 +163,16 @@ 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,
+
+             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);
         }
     });