use swizzle syntax

v_shader.glsl

@@ -8,6 +8,6 @@ varying vec3 normal_i;
 void main(void)
 {
     gl_Position = gl_ModelViewProjectionMatrix * vec4(pos, 1);
-    pos_i = vec3(gl_Position.x, gl_Position.y, gl_Position.z);
+    pos_i = gl_Position.xyz;
     normal_i = gl_NormalMatrix * normal;
 }