[gnome-shell/gnome-3-36] st-label: Keep labels fully pre-rendered on the GPU

[Robert Mader <rmader src gnome org>]

commit 96f1d1b08db550bd51374c081f8729383a7065a0
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Wed Jun 24 16:00:17 2020 +0800

    st-label: Keep labels fully pre-rendered on the GPU
    
    The performance of the icon grid was being hindered by a large number
    of primitives (a few hundred) being copied from the CPU to the GPU on
    each frame. This was first noticed in mutter#971 but we failed to
    investigate all the issues at the time.
    
    You can also see the high number using `COGL_DEBUG=batching` or
    `COGL_DEBUG=disable-texturing`. So now it's obvious that high number is
    every letter of every label being uploaded as a separate quad. Let's not
    do that and instead treat the whole label as a single quad/texture.
    
    Measured performance on an i7-7700 at UHD 3840x2160:
    
    Journal entries per frame on the icon grid:
     * Before: 288 (18 KB copied from CPU to GPU)
     * After:   73 ( 4 KB copied from CPU to GPU)
    
    Spring animation:
     * Before: 20-30 FPS, avg 22/peak 45 milliseconds per frame
     * After:  30-40 FPS, avg 14/peak 28 milliseconds per frame
    
    Scrolling the icon grid:
     * Before: 15 FPS, 50 milliseconds per frame
     * After:  30 FPS, 28 milliseconds per frame
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1329
    
    (cherry picked from commit ae338af1e80ed89eb624a36a666894c986573c76)

 src/st/st-label.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/src/st/st-label.c b/src/st/st-label.c
index 947c8ec861..3f82a2c152 100644
--- a/src/st/st-label.c
+++ b/src/st/st-label.c
@@ -294,6 +294,7 @@ st_label_class_init (StLabelClass *klass)
 static void
 st_label_init (StLabel *label)
 {
+  ClutterActor *actor = CLUTTER_ACTOR (label);
   StLabelPrivate *priv;
 
   label->priv = priv = st_label_get_instance_private (label);
@@ -305,7 +306,10 @@ st_label_init (StLabel *label)
   label->priv->shadow_width = -1.;
   label->priv->shadow_height = -1.;
 
-  clutter_actor_add_child (CLUTTER_ACTOR (label), priv->label);
+  clutter_actor_add_child (actor, priv->label);
+
+  clutter_actor_set_offscreen_redirect (actor,
+                                        CLUTTER_OFFSCREEN_REDIRECT_ALWAYS);
 }
 
 /**