[Linux] Setting fullscreen game resolution below set window resolution does not stretch the game across the window.

If the resolution of the game is set to something lower than the screen resolution and then set to fullscreen the game will not stretch to fit the screen and portions will be left black.

1. Open UE4Editor (any project)
2. In the level Blueprint add Key press event and wire into Execute Console Command node
3. Determine resolution of the monitor (tested on 1680x1050)
4. Set console command to a lower value (setres 1024x768f)
5. Play in standalone mode
6. Press button to execute the setres command

Result:
The fullscreen game will show black areas without stretching the game to fill the screen.

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