I have Gamecenter, I watched the game on the Detroit feed, and decided to check out the feed from Calgary to see how the replays compared, and they both showed the same angle, the Detroit feed didn't show it any closer or more clear than what you saw. It was very close, hard to tell on TV, but you have to give the linesman the benefit of doubt there, he was staring at the puck to make that call and it is his job... calls like that happen. If it was offside, it was barely over the line, and if it was onside, it was kept in just before the puck completely crossed the line. Not easy to make those call from the ice in real time.
But Mrazek also gave up on the play when he heard a whistle well before the puck went into the net, IMO. Looked like he wasn't ready for the shot because he heard the whistle for the offside and was surprised to see the puck coming at him when it went in the net, so even if they didn't call it offside, Mrazek might have made the save had there not been a whistle.