[gtk/wip/chergert/glyphy] gsk/gl: fix output color of glyphy fragment shader

[Christian Hergert <chergert src gnome org>]

commit 7a1b50ff4ed54e5f61c748309380936b89ece2cb
Author: Christian Hergert <chergert redhat com>
Date:   Sat Mar 19 21:39:26 2022 -0700

    gsk/gl: fix output color of glyphy fragment shader
    
    This copied more or less what the coloring vertex shader was doing in
    that we premultiply alpha. That changes how we apply alpha in the fragment
    shader to match.
    
    This fixes a white halo around the fonts.

 gsk/gl/resources/glyphy.fs.glsl | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)
---
diff --git a/gsk/gl/resources/glyphy.fs.glsl b/gsk/gl/resources/glyphy.fs.glsl
index 691ad77b1f..4e41b67bef 100644
--- a/gsk/gl/resources/glyphy.fs.glsl
+++ b/gsk/gl/resources/glyphy.fs.glsl
@@ -46,8 +46,6 @@ main()
   vec2 dpdy = dFdy (p);
   float m = length (vec2 (length (dpdx), length (dpdy))) * SQRT2_2;
 
-  vec4 color = final_color;
-
   float gsdist = glyphy_sdf (p, gi.nominal_size GLYPHY_DEMO_EXTRA_ARGS);
   float sdist = gsdist / m * u_contrast;
 
@@ -60,9 +58,10 @@ main()
     float alpha = antialias (-sdist);
     if (u_gamma_adjust != 1.)
       alpha = pow (alpha, 1./u_gamma_adjust);
-    color = vec4 (color.rgb,color.a * alpha);
+
+    gskSetOutputColor(final_color * alpha);
   } else {
-    color = vec4 (0,0,0,0);
+    vec4 color = vec4 (0,0,0,0);
 
     // Color the inside of the glyph a light red
     color += vec4 (.5,0,0,.5) * smoothstep (1., -1., sdist);
@@ -82,7 +81,7 @@ main()
     glyphy_arc_list_t arc_list = glyphy_arc_list (p, gi.nominal_size GLYPHY_DEMO_EXTRA_ARGS);
     // Color the number of endpoints per cell blue
     color += vec4 (0,0,1,.1) * float(arc_list.num_endpoints) * 32./255.;
-  }
 
-  gskSetOutputColor(color);
+    gskSetOutputColor(color);
+  }
 }