title: Apply post processing shader
body: <p>I have a Grayscale shader that work when applied to a material.
  I'd like to apply it to what the camera renders.</p>
  
  <p>I found some tutorial speaking about
  void OnRenderImage (RenderTexture source, RenderTexture destination)</p>
  
  <p>but I wasn't able to make it work with mine.</p>
  
  <p>The shader :</p>
  
  <pre><code>Shader "Custom/Grayscale" {
  
  SubShader {
      Pass{
  
          CGPROGRAM
  
          #pragma exclude_renderers gles
          #pragma fragment frag
  
          #include "UnityCG.cginc"
  
          uniform sampler2D _MainTex;
  
          fixed4 frag(v2f_img i) : COLOR{
              fixed4 currentPixel = tex2D(_MainTex, i.uv);
  
              fixed grayscale = Luminance(currentPixel.rgb);
              fixed4 output = currentPixel;
  
              output.rgb = grayscale;
              output.a = currentPixel.a;
  
              //output.rgb = 0;
  
              return output;
          }
  
          ENDCG
      }
  
  }
  
  FallBack "VertexLit"
  }
  </code></pre>
  
  <p>And the script attached to the camera :</p>
  
  <pre><code>using UnityEngine;
  using System.Collections;
  
  public class Grayscale : ImageEffectBase {
  
  void OnRenderImage (RenderTexture source, RenderTexture destination) {
      Graphics.Blit (source, destination, material);
  }
  }
  </code></pre>
  
  <p>Any suggestions ?
  Many thanks !</p>