FullScreenQuad.cs

// Attach this script to the camera
using UnityEngine;

public class FullScreenQuad : MonoBehaviour
{
    public Material fullScreenStencilMat;

    private void OnPostRender()
    {
        fullScreenStencilMat.SetPass(0);
        GL.PushMatrix();
        GL.LoadPixelMatrix(0, 1, 0, 1);
        GL.modelview = Matrix4x4.identity;
        GL.Color(Color.white);
        GL.Begin(GL.QUADS);
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(0, 1, 0);
        GL.Vertex3(1, 1, 0);
        GL.Vertex3(1, 0, 0);
        GL.End();
        GL.PopMatrix();

    }
}