The elevated horror-western “ The Trek ,” set in Africa’s barren Kalahari desert, isn’t an easy watch. But it is a beautiful one. As one of South Africa’s most daring new voices, Meekaeel Adam makes his directorial debut in this gripping story, set in 1846, of a Dutch-Afrikaans family and their British benefactor who make the perilous journey across the Kalahari Desert, watched over by two shapeshifting spirits. “The Trek,” produced by The Department of Special Projects, stars Morne Visser, Rob