Hello there, I am so sorry to do this necropost but would this work? It does work in Windows.
sf::RenderWindow window(sf::VideoMode(640, 320), "Proyecto001",sf::Style::Fullscreen);
view.reset(sf::FloatRect(1, 1, 640, 320));
window.setView(view);
This creates a "camera" that stares only at whatever you chose, in this case the first 640x320 pixels, and stretches them to the full screen.
This would solve the problem, however, this will not respect the aspect ratio, obviously.
Greetings!