Stella (Lesley Manville) and Gerry (Ciarán Hinds), the couple at the center of the new film “Midwinter Break,” appear perfectly comfortable in their marriage. They have been at it for decades, navigating a move to a new country, changing careers and raising a son who now has a family of his own. But every marriage has its tensions, the eternal sticking points that both sides choose to accept (or tolerate) for the good of the union. Gerry’s drinking is one; Stella’s devout Catholicism (and Gerry’s lack thereof) is another.