Enable black borders on widescreen display

[Dustin Lange]

Hello,

I have a notebook with widescreen display (Dell Latitude D620) and want
to enable the following configuration (for presentations):

1. An external screen is connected; both screens use a resolution of
1024x768.
2. The widescreen display should display black borders on the left and
right hand side. Reason: The image should not be stretched to fit the
widescreen display.

The first requirement seems to be no problem. I can use the following 
xrandr command to set the resolution properly:

xrandr --output LVDS --mode 1024x768 --output VGA --mode 1024x768

But how can I convince xrandr to display the black borders and not to 
stretch the image on the widescreen display?

Thanks,
Dustin