RenderTextures and FullScreen Quads

Hey all,

Trying to write a small script that when attached to a camera will allow me to control the size of the rendered screen (so that I can render to a smaller texture, then scale that up to the screen size for performance).

EDIT: The new code bellow currently renders the camera, and uses the OnRenderImage() to draw it to a fullscreen quad (through to another render texture). Both output image files display expected results, the scene as it should be, and an inverted colour version.

 using UnityEngine;
 using System.Collections;
 
 public class RenderBufferControl : BaseObject
 {
     private Material mat;
     private RenderTexture rTex;
     private Camera _cam;
 
     protected override void CustomAwake () 
     {
         _cam = GetComponent<Camera>();
 
         mat = new Material (
             "Shader \"Hidden/Invert\" {" +
             "SubShader {" +
             "    Pass {" +
             "        ZTest Always Cull Off ZWrite Off" +
             "        SetTexture [_RenderTex] { combine one-texture }" +
             "    }" +
             "}" +
             "}"
         );
 
         rTex = new RenderTexture(1024, 1024, 24);
 
         _cam.targetTexture = rTex;
     }
 
     void OnRenderImage (RenderTexture source, RenderTexture dest) 
     {
         RenderTexture.active = dest;
         source.SetGlobalShaderProperty ("_RenderTex");
 
         DrawFullScreenQuad(mat);
 
         // Output source and destination to images to confirm their contents.
         Util.SaveImage(source, "Source.png");
         Util.SaveImage(dest, "Dest.png");
     }
 
     public static void DrawFullScreenQuad(Material material)
     {
         GL.PushMatrix();
         GL.LoadOrtho();
         
         int i = 0;
         while (i < material.passCount)
         {
             material.SetPass(i);
             GL.Begin(GL.QUADS);
             GL.Color(Color.white);
             GL.TexCoord2(0, 0);
             GL.Vertex3(0, 0, 0.1f);
 
             GL.TexCoord2(1, 0);
             GL.Vertex3(1, 0, 0.1f);
 
             GL.TexCoord2(1, 1);
             GL.Vertex3(1, 1, 0.1f);
 
             GL.TexCoord2(0, 1);
             GL.Vertex3(0, 1, 0.1f);
             GL.End();
             ++i;
         }
 
         GL.PopMatrix();
     }
 }

The only issue now is that I need to be drawing the fullscreen quad to the actual screen, not just to another render texture. I'm not entirely sure how to do this, could anyone suggest a way of achieving this?

EDIT: Currently I've managed to get it working by setting the source render texture in OnRenderImage() as the texture for a material on a plane set in front of an orthographic camera that is rendered after the main camera. this seems to work, but I'd still like to know if this is the best way to do it, or if there are any other methods to use.