Specify layout index in shader

shaders/thumb.frag

@@ -4,7 +4,7 @@ uniform sampler2DArray qt_Texture0;
 uniform vec3 mtf_param;
 uniform bool invert;
 in vec3 qt_TexCoord0;
-out vec4 color;
+layout(location = 0) out vec4 color;
 
 vec4 MTF(vec4 x, vec3 m)
 {