As King Lear, Glenda Jackson enters with a casual swagger, giving off a scent of power that’s lasted a lifetime. But Lear is portrayed by an 82-year-old woman, lean and light and, at 5’6, dwarfed by the dignitaries and daughters who share the stage in the opening scene at the Cort Theater. If Jackson’s performance turns this inventive but imperfect production into a must-see of the Broadway Spring season, it’s not just because of her impressive stamina and control. It’s also the appearance of physical fragility that helps make her Lear stand out. …DIrector Sam Gold creates some memorable theatrics in his King Lear, some of which illuminate the text for us, little of which undermine it for me.