How has the movie theater business changed in the last decade? Sure, there’s been streaming, declining ticket sales and a wave of recliners, alcohol and other new offerings to entice customers back to multiplexes. But on opening weekend of “Avengers: Endgame,” all those stormy warnings of industry decline never felt more distant. In theaters around the world, from rural markets to the biggest cities, screenings for the Marvel Studios blockbuster were selling out just minutes after advance tickets went on sale weeks before release. Hundreds of the largest theaters in the country stayed open around the clock for the whole weekend, and thousands more introduced new screenings once all their available showtimes sold out.