Render on an image before sending to screen

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You want "letterboxed" graphics in fullscreen mode? I'll write an example as soon as I get home from work

I'm not sure what you name letterboxed graphies. I want to be able to run my game in fullscreen and the aspect ratio is respected what ever is the aspect ratio of the screen, displaying margins, vertical  or horizontal if needed to center the game display area. 
Like seeing cinema 16/9 films on old 4/3 tv screens.
Thank you.