[mutter/wip/chergert/identity-matrix] clutter/actor: avoid transform node for identity matrix

[Christian Hergert <chergert src gnome org>]

commit cfc348d69c4045ecb8168cf69d391e69e1417745
Author: Christian Hergert <chergert redhat com>
Date:   Wed Feb 19 21:22:13 2020 -0800

    clutter/actor: avoid transform node for identity matrix
    
    If the transform matrix is an identity, then positioning wont change and
    we can avoid creating the transform node altogether. This is based on
    a similar find in GTK today while reducing temporary allocations.
    
    This cuts the number of transforms created in clutter_actor_paint() by
    about half under light testing of GNOME Shell from 6.8% to 2.4% of
    allocations.
    
    Before:
    
        ALLOCATED      TOTAL    FUNCTION
    [   20.4 MiB] [  21.20%]    clutter_actor_paint
    [   11.0 MiB] [  11.45%]      clutter_paint_node_paint
    [    6.6 MiB] [   6.84%]      clutter_transform_node_new
    [    2.5 MiB] [   2.61%]      clutter_actor_node_new
    
    After:
    
        ALLOCATED      TOTAL    FUNCTION
    [   33.4 MiB] [  24.12%]    clutter_actor_paint
    [   26.2 MiB] [  18.91%]      clutter_paint_node_paint
    [    3.4 MiB] [   2.43%]      clutter_actor_node_new
    [    3.3 MiB] [   2.41%]      clutter_transform_node_new
    
    Allocation amounts will have differed due to different amounts of running
    time, but the % of allocations has now dropped below
    clutter_actor_node_new() which should be expected.
    
    https://gitlab.gnome.org/GNOME/mutter/issues/1056

 clutter/clutter/clutter-actor.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/clutter/clutter/clutter-actor.c b/clutter/clutter/clutter-actor.c
index 610741abc..163d30d43 100644
--- a/clutter/clutter/clutter-actor.c
+++ b/clutter/clutter/clutter-actor.c
@@ -3997,11 +3997,14 @@ clutter_actor_paint (ClutterActor        *self,
 
       clutter_actor_get_transform (self, &transform);
 
-      transform_node = clutter_transform_node_new (&transform);
-      clutter_paint_node_add_child (transform_node, root_node);
-      clutter_paint_node_unref (root_node);
+      if (!cogl_matrix_is_identity (&transform))
+        {
+          transform_node = clutter_transform_node_new (&transform);
+          clutter_paint_node_add_child (transform_node, root_node);
+          clutter_paint_node_unref (root_node);
 
-      root_node = g_steal_pointer (&transform_node);
+          root_node = g_steal_pointer (&transform_node);
+        }
 
 #ifdef CLUTTER_ENABLE_DEBUG
       /* Catch when out-of-band transforms have been made by actors not as part