Two young Texans are jailed in Shreveport -- after allegedly breaking into a car at a downtown parking garage early this morning.  They were spotted, in the act, by a police officer but, as he started toward them, they jumped into a white Lincoln Town Car and drove off.  Other officers began chasing that car to Murphy Street, Hearne Avenue, and Greenwood Road.  Just east of the American Rose Center, officers deployed spike strips to deflate the car's tires.  When it stopped, the two occupants jumped out and tried to run but a police K9 ended their flight.  18-year-old Justin Johnson from Harleton, Texas, was treated at LSU Hospital for a dog bite to his leg, then booked into the Shreveport City Jail on charges of burglary and aggravated flight from an officer.  A 16-year-old boy from Marshal, Texas, was booked into the Caddo Juvenile Detention Center on similar charges.