Oregon-set folk horror feature “Antlers,” finally being released after a long pandemic-induced delay, resembles what one might find in a rusty cauldron after a disorderly witch mixed an array of incongruent ingredients for a potion. Chunks of childhood trauma, a dash of the opioid crisis, a few drops of environmental distress, and Native American mythology swim together in a foggy concoction of a plot without meaningfully merging.