Standing amid the stars of “Thunderbolts,” “Fantastic Four” and some of the OG “Avengers” as they posed for a massive group shot during Marvel’s super-sized Hall H panel on Saturday at Comic-Con, was Robert Downey Jr. in the middle. It was a visceral reminder that the man who launched the MCU is once again its center of gravity — this time trading Tony Stark’s armor for Doctor Doom’s mask and cloak.