NEW YORK — A solemn line of fellow police officers and everyday New Yorkers, some standing in the cold for three hours Thursday, filed inside St. Patrick’s Cathedral to offer their prayers for slain NYPD Officer Jason Rivera. The casket bearing the 22-year-old officer reached the venerable Midtown church around 11:30 a.m., with family members arriving prior to an afternoon viewing open to the public. Photos of Rivera from throughout his young life decorated the altar around an open casket adorned with red roses while his widow and parents sat together as the mourners poured in from the Manhatt…