CGI loses the day in Steven Spielberg’s The BFG, a partly motion-captured, eco-minded adaptation of Roald Dahl’s adored children’s book that leans so heavily on green-screen trickery that even Mark Rylance’s kind eyes — squinting out from that computer-generated abyss — can’t save it from mediocrity. The plotline of a friendly, dream-blowing giant who takes an orphaned girl under his wing has the great director’s name all over it, but those expecting something for the whole family here may feel a little let down. That is, unless fart-propelled corgis fizzing around Buckingham Palace is your kind of thing. (However, that’s nowhere near as funny as it sounds.)