Set the rendering viewport.

All rendering is constrained to be inside the passed pixelRect.
If the Viewport is modified, all the rendered content inside of it gets stretched.

	// Draw a red Quad that covers all the view port, and the when space is pressed
	// the viewport gets expanded to the whole screen and stretch the contents inside
	var mat : Material;
	private var stretch : boolean = false;
	function Update() {
		if(Input.GetKeyDown(KeyCode.Space)) {
			if(stretch) {
				stretch = false;
			} else {
				stretch = true;
			}
		}
	}
	function OnPostRender() {
		if (!mat) {
			Debug.LogError("Please Assign a material on the inspector");
			return;
		}
		GL.PushMatrix();
		mat.SetPass(0);
		GL.LoadPixelMatrix();
		if(stretch) {
			GL.Viewport(Rect(0,0,Screen.width,Screen.height));
		} else {
			GL.Viewport(Rect(0,0,Screen.width/2,Screen.height));
		}
		GL.Color(Color.red);
		GL.Begin(GL.QUADS);
		GL.Vertex3(0,0,0);
		GL.Vertex3(0,Screen.height,0);
		GL.Vertex3(Screen.width,Screen.height,0);
		GL.Vertex3(Screen.width,0,0);
		GL.Color(Color.yellow);
		GL.End();
		GL.Begin(GL.TRIANGLES);
		GL.Vertex3(Screen.width/2,Screen.height/4,1);
		GL.Vertex3(Screen.width/4,Screen.height/2,1);
		GL.Vertex3(Screen.width - Screen.width/4,Screen.height/2,1);
		GL.End();
		GL.PopMatrix();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Material mat;
    private bool stretch = false;
    void Update() {
        if (Input.GetKeyDown(KeyCode.Space))
            if (stretch)
                stretch = false;
            else
                stretch = true;
        
    }
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadPixelMatrix();
        if (stretch)
            GL.Viewport(new Rect(0, 0, Screen.width, Screen.height));
        else
            GL.Viewport(new Rect(0, 0, Screen.width / 2, Screen.height));
        GL.Color(Color.red);
        GL.Begin(GL.QUADS);
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(0, Screen.height, 0);
        GL.Vertex3(Screen.width, Screen.height, 0);
        GL.Vertex3(Screen.width, 0, 0);
        GL.Color(Color.yellow);
        GL.End();
        GL.Begin(GL.TRIANGLES);
        GL.Vertex3(Screen.width / 2, Screen.height / 4, 1);
        GL.Vertex3(Screen.width / 4, Screen.height / 2, 1);
        GL.Vertex3(Screen.width - Screen.width / 4, Screen.height / 2, 1);
        GL.End();
        GL.PopMatrix();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mat as Material

	private stretch as bool = false

	def Update() as void:
		if Input.GetKeyDown(KeyCode.Space):
			if stretch:
				stretch = false
			else:
				stretch = true

	def OnPostRender() as void:
		if not mat:
			Debug.LogError('Please Assign a material on the inspector')
			return
		GL.PushMatrix()
		mat.SetPass(0)
		GL.LoadPixelMatrix()
		if stretch:
			GL.Viewport(Rect(0, 0, Screen.width, Screen.height))
		else:
			GL.Viewport(Rect(0, 0, (Screen.width / 2), Screen.height))
		GL.Color(Color.red)
		GL.Begin(GL.QUADS)
		GL.Vertex3(0, 0, 0)
		GL.Vertex3(0, Screen.height, 0)
		GL.Vertex3(Screen.width, Screen.height, 0)
		GL.Vertex3(Screen.width, 0, 0)
		GL.Color(Color.yellow)
		GL.End()
		GL.Begin(GL.TRIANGLES)
		GL.Vertex3((Screen.width / 2), (Screen.height / 4), 1)
		GL.Vertex3((Screen.width / 4), (Screen.height / 2), 1)
		GL.Vertex3((Screen.width - (Screen.width / 4)), (Screen.height / 2), 1)
		GL.End()
		GL.PopMatrix()