Pro-government forces have gotten some backup in their battle for the Baiji refinery, after suffering heavy losses during a late-April offensive by the so-called Islamic State (IS) militant group.IS fighters still control about half of the 36-square-kilometer refinery campus, according to several security officials in the area.Security forces had been stranded with limited ammunition and supplies, and reinforcements were unable to help: IS fighters had rigged the roads leading to the ref…