AlphaBlit.cs

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class AlphaBlit : MonoBehaviour
{
    public Material alphaBlitMat;
    public RenderTexture rt;

    void OnRenderImage(RenderTexture source, RenderTexture destination)
    {
        Graphics.Blit(rt, destination, alphaBlitMat);
    }
}