Houston’s downtown skyline is ever-changing; as towering skyscrapers go up, each seemingly tests the limits of construction and engineering. No one likely has had a bigger impact on that outline than Gerald D. Hines and his Hines real-estate-development firm that has built significant structures in Houston and elsewhere for more than 60 years. Hines died Aug. 23 at the age of 95 in his home in New Haven, Conn.