Better when actually include the shaders.

data/shaders/sky.frag

@@ -0,0 +1,17 @@
+#version 330
+uniform samplerCube tex;
+uniform mat4 InvProjView;
+uniform vec2 screen;
+
+in vec2 uv;
+out vec4 FragColor;
+
+void main(void)
+{
+    vec3 eyedir = gl_FragCoord.xyz / vec3(screen, 1.);
+    eyedir = 2.0 * eyedir - 1.0;
+	vec4 tmp = (InvProjView * vec4(eyedir, 1.));
+	eyedir = tmp.xyz / tmp.w;
+    vec4 color = texture(tex, eyedir);
+    FragColor = vec4(color.xyz, 1.);
+}