Fix shader on OpenGL

Varying name has to match.

boxblur.frag

@@ -50,7 +50,7 @@
 
 #version 440
 
-layout(location = 0) in vec2 textureCoord;
+layout(location = 0) in vec2 qt_TexCoord0;
 layout(location = 0) out vec4 fragColor;
 
 layout(std140, binding = 0) uniform buf {
@@ -66,7 +66,7 @@ void main(void)
     vec3 sum = vec3(0.0, 0.0, 0.0);
     for (int x = -radius; x <= radius; ++x) {
         for (int y = -radius; y <= radius; ++y) {
-            vec2 c = textureCoord + vec2(x, y) * pixelStep;
+            vec2 c = qt_TexCoord0 + vec2(x, y) * pixelStep;
             sum += texture(src, c).rgb;
         }
     }