PARIS — A world premiere, Morgane Polanski’s “The Stroke,” starring fellow “Vikings” co-star Edvin Endre, bowed Friday on MyFrenchFilmFestival, the UniFrance global online showcase of French films. Polanski’s follow-up to “The Understudy,” her debut short film as a director, “The Stroke” also suggests a budding auteur. Both films focus on obsessions.