Fix shader compile error due to missing variable.

src/Shaders/background.fragment.fx

@@ -6,6 +6,8 @@ precision highp float;
 
 #include<__decl__backgroundFragment>
 
+#define RECIPROCAL_PI2 0.15915494
+
 // Constants
 uniform vec3 vEyePosition;
 
@@ -286,4 +288,4 @@ vec4 color = vec4(finalColor, finalAlpha);
 #endif
 
     gl_FragColor = color;
-}
+}