For starters, it’s not simply absent one day and fully present the next. In film and TV, portrayals of dementia leave a lot to be desired. There’s a lazy, increasingly common trope in horror movies of a grandparent, usually a grandma, with Alzheimer’s who, lost to the disease, becomes “Exorcist”-level demonic. Most notably, there’s “The Taking of Deborah Logan,” a 2014 found-footage horror movie about a possessed nana with Alzheimer’s whose behavior grows increasingly disturbing — she opens and