Swiss Films Previews, the only spread of national movies at Switzerland’s Visions du Réel, the country’s leading doc festival. Presented via excerpts at a two-hour showcase on Wednesday, three further titles – “Kalari – the Martial Art of Female Power,” “The Boy from the River Drina” and “Spheres” – underscored the strength in depth of documentary filmmaking in Switzerland and at least in this year’s Previews, a leitmotif.