Add two-sided rendering to the fragment shader.

android/AndroidOpenGLESLessons/res/raw/per_pixel_fragment_shader_no_tex.glsl

@@ -18,7 +18,13 @@ void main()
 
 	// Calculate the dot product of the light vector and vertex normal. If the normal and light vector are
 	// pointing in the same direction then it will get max illumination.
-    float diffuse = max(dot(v_Normal, lightVector), 0.0);               	  		  													  
+	float diffuse;
+
+	if (gl_FrontFacing) {
+        diffuse = max(dot(v_Normal, lightVector), 0.0);
+    } else {
+    	diffuse = max(dot(-v_Normal, lightVector), 0.0);
+    }               	  		  													  
 
 	// Add attenuation. 
     diffuse = diffuse * (1.0 / (1.0 + (0.10 * distance)));