public class Framer : MonoBehaviour
{
    public Links links;
    public Camera camera_in;
    public Camera camera_preview;
    public Material frameMat;
    float offset = 0.002f;

    void OnPostRender()
    {
        if (camera_in.gameObject.active)
            Draw(camera_in);
        if (links.guiManager.onPreview && camera_preview.gameObject.active)
            Draw(camera_preview);
    }

    void Draw(Camera cam)
    {
        GL.PushMatrix();
        frameMat.SetPass(0);
        GL.LoadOrtho();
        GL.Begin(GL.QUADS);
        GL.Color(Color.white);

        GL.Vertex3(cam.rect.xMin - offset, cam.rect.yMin - offset, 0.3f);
        GL.Vertex3(cam.rect.xMin - offset, cam.rect.yMax + offset, 0.3f);
        GL.Vertex3(cam.rect.xMax + offset, cam.rect.yMax + offset, 0.3f);
        GL.Vertex3(cam.rect.xMax + offset, cam.rect.yMin - offset, 0.3f);

        GL.End();
        GL.PopMatrix();
    }
}