[gtk: 5/15] shadertoy demo: Fix GLSL on GLES

[Alexander Larsson <alexl src gnome org>]

commit 16cdb33c6c17615efaa57fd04e2ac14180bf68fa
Author: Alexander Larsson <alexl redhat com>
Date:   Wed Sep 23 17:38:03 2020 +0200

    shadertoy demo: Fix GLSL on GLES
    
    I was getting "assignment to varying fragColor" errors

 demos/gtk-demo/gtkshadertoy.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/demos/gtk-demo/gtkshadertoy.c b/demos/gtk-demo/gtkshadertoy.c
index 866bd69481..7d8c4926f6 100644
--- a/demos/gtk-demo/gtkshadertoy.c
+++ b/demos/gtk-demo/gtkshadertoy.c
@@ -56,13 +56,15 @@ const char *fragment_prefix =
   "uniform float     iSampleRate;           // sound sample rate (i.e., 44100)\n"
   "\n"
   "in vec2 fragCoord;\n"
-  "out vec4 fragColor;\n";
+  "out vec4 vFragColor;\n";
 
 
 // Fragment shader suffix
 const char *fragment_suffix =
   "    void main() {\n"
-  "        mainImage(fragColor, fragCoord);\n"
+  "        vec4 c;\n"
+  "        mainImage(c, fragCoord);\n"
+  "         vFragColor = c;\n"
   "    }\n";
 
 typedef struct {