It is ironic, in a film of precious little irony, that Marie Kreutzer ‘s intelligently made but unremittingly bleak “ Gentle Monster ” — the Austrian director’s Cannes competition-selected follow-up to her Un Certain Regard prizewinner “Corsage” — should not only begin and end with a trampoline, but should to some degree pivot on the uncomplicatedly happy image of a little boy somersaulting and bouncing on it. Plotting a linear, sinking trajectory, Kreutzer’s discomfiting film describes no such buoyant highs and lows. Here, what goes up must come down and down and still further down. The little boy is Johnny (Malo Blanchet), the son of young parents Lucy ( Léa Seydoux ) and Philip (Laurence Rupp). Lucy is French, and an avant-garde musician who performs deconstructed covers of pop songs, exclusively by male artists, played on an array of unusual, seemingly self-designed instruments.…