[mutter] shaped-texture: Use the REPLACE combine function on opaque areas

[Robert Mader <rmader src gnome org>]

commit e5542c3210229d944f66c030dc26fe4e9d010607
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu Jun 25 19:00:29 2020 +0800

    shaped-texture: Use the REPLACE combine function on opaque areas
    
    We were setting the pipeline colour to all white (1.0, 1.0, 1.0, 1.0)
    and so the default layer combine function multiplied each pixel
    (R, G, B, A) by all ones. Obviously multiplying by one four times per
    pixel is a waste of effort so we remove the colour setting *and* set
    the layer combine function to a trivial shader that will ignore whatever
    the current pipeline colour is set to. So now we do **zero** multiplies
    per pixel.
    
    On an i7-7700 at UHD 3840x2160 this results in 5% faster render times
    and 10% lower power usage (says intel_gpu_top). The benefit is probably
    much higher for virtual machines though, as they're no longer being
    asked to do CPU-based math on every pixel of a window.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1331

 src/compositor/meta-shaped-texture.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)
---
diff --git a/src/compositor/meta-shaped-texture.c b/src/compositor/meta-shaped-texture.c
index ed8e3c0bcf..630efb8df4 100644
--- a/src/compositor/meta-shaped-texture.c
+++ b/src/compositor/meta-shaped-texture.c
@@ -408,17 +408,14 @@ get_unblended_pipeline (MetaShapedTexture *stex,
                         CoglContext       *ctx)
 {
   CoglPipeline *pipeline;
-  CoglColor color;
 
   if (stex->unblended_pipeline)
     return stex->unblended_pipeline;
 
   pipeline = cogl_pipeline_copy (get_base_pipeline (stex, ctx));
-  cogl_color_init_from_4ub (&color, 255, 255, 255, 255);
-  cogl_pipeline_set_blend (pipeline,
-                           "RGBA = ADD (SRC_COLOR, 0)",
-                           NULL);
-  cogl_pipeline_set_color (pipeline, &color);
+  cogl_pipeline_set_layer_combine (pipeline, 0,
+                                   "RGBA = REPLACE (TEXTURE)",
+                                   NULL);
 
   stex->unblended_pipeline = pipeline;