cogl: Port Bitmap away from CoglObject

We still need to use set_qdata_full as CoglBitmapPixbuf would free
the data itself by unrefing the pixbuf

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3193>

src/backends/native/meta-onscreen-native.c

@@ -865,7 +865,7 @@ copy_shared_framebuffer_cpu (CoglOnscreen                        *onscreen,
                                                  dumb_bitmap))
     g_warning ("Failed to CPU-copy to a secondary GPU output");
 
-  cogl_object_unref (dumb_bitmap);
+  g_object_unref (dumb_bitmap);
 
   secondary_gpu_state->cpu.current_dumb_fb = buffer_dumb;