Reverse normals on GLSL

Hi,

I'm trying to modify the Google Carboard VR Unity shader to flip rendered texture inside the mesh, so I should invert normals (I don't want to modify the mesh), I added a checkbox/float variable but I'm not able to modify the vertex/main() body to do the operation, I added also the .ifdef area, anyone could help ? thank you.

Code (CSharp):

1. Shader "GoogleVR/Video Unlit Shader" {
2.   Properties {
3.     _Gamma ("Video gamma", Range(0.01,3.0)) = 1.0
4.     _MainTex ("Base (RGB)", 2D) = "white" {}
5.     [KeywordEnum(None, TopBottom, LeftRight)] _StereoMode ("Stereo mode", Float) = 0
6.     [Toggle(FLIP_X)] _FlipX ("Flip X", Float) = 0
7.     [Toggle(FLIP_NORMALS)] _FlipNormals ("Flip Normals", Float) = 0
8.   }
9.  
10.   SubShader {
11.     Pass {
12.       Tags { "RenderType" = "Opaque" }
13.  
14.       Lighting Off
15.       Cull Off
16.  
17.       GLSLPROGRAM
18.         #pragma only_renderers gles gles3
19.         #extension GL_OES_EGL_image_external : require
20.         #extension GL_OES_EGL_image_external_essl3 : enable
21.  
22.         #pragma multi_compile ___ _STEREOMODE_TOPBOTTOM _STEREOMODE_LEFTRIGHT
23.         #pragma multi_compile ___ FLIP_X
24.         #pragma multi_compile ___ FLIP_NORMALS
25.  
26.         precision mediump int;
27.         precision mediump float;
28.  
29.         #ifdef VERTEX
30.           uniform mat4 video_matrix;
31.           uniform int unity_StereoEyeIndex;
32.           varying vec2 uv;
33.  
34.           void main() {
35.             gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
36.             vec4 untransformedUV = gl_MultiTexCoord0;
37.  
38.            #ifdef FLIP_NORMALS
39.               *************************** what to do here ? *********
40.             #endif  // END
41.  
42.             #ifdef FLIP_X
43.               untransformedUV.x = 1.0 - untransformedUV.x;
44.             #endif  // FLIP_X
45.             #ifdef _STEREOMODE_TOPBOTTOM
46.               untransformedUV.y *= 0.5;
47.               if (unity_StereoEyeIndex == 0) {
48.                 untransformedUV.y += 0.5;
49.               }
50.             #endif  // _STEREOMODE_TOPBOTTOM
51.             #ifdef _STEREOMODE_LEFTRIGHT
52.               untransformedUV.x *= 0.5;
53.               if (unity_StereoEyeIndex != 0) {
54.                 untransformedUV.x += 0.5;
55.               }
56.             #endif  // _STEREOMODE_LEFTRIGHT
57.  
58.             uv = (video_matrix * untransformedUV).xy;
59.           }
60.         #endif  // VERTEX
61.  
62.         #ifdef FRAGMENT
63.           vec3 gammaCorrect(vec3 v, float gamma) {
64.             return pow(v, vec3(1.0/gamma));
65.           }
66.  
67.           // Apply the gamma correction.  One possible optimization that could
68.           // be applied is if _Gamma == 2.0, then use gammaCorrectApprox since sqrt will be faster.
69.           // Also, if _Gamma == 1.0, then there is no effect, so this call could be skipped all together.
70.           vec4 gammaCorrect(vec4 v, float gamma) {
71.             return vec4(gammaCorrect(v.xyz, gamma), v.w);
72.           }
73.  
74.           uniform float _Gamma;
75.           uniform samplerExternalOES _MainTex;
76.           varying vec2 uv;
77.  
78.           void main() {
79.             gl_FragColor = gammaCorrect(texture2D(_MainTex, uv), _Gamma);
80.           }
81.         #endif  // FRAGMENT
82.       ENDGLSL
83.     }
84.   }
85.   Fallback "Unlit/Texture"
86. }