In press notes for the new film “When Fall Is Coming,” acclaimed French writer/director François Ozon asks a ridiculous yet reasonable question: “When we cook wild mushrooms, are we not, more or less subconsciously, trying to get rid of someone?” A bit of the same mix of the farcical and the perceptive describes the film itself, focused as it is on an elderly grandmother who may or may not be involved in her daughter’s death.