(Image credit: Valve, Breadworks) My high school years were spent pumping thousands of hours into games like Killing Floor 2 and Team Fortress 2, so I'm glad a mod like the upcoming Mann Versus Zombies—a total conversion which turns TF2 into a zombie-blasting horde survival bonanza—didn't come out during my high school years. I probably would not have graduated. The details are on its Steam page , and if it weren't for the yellow wrench banner informing me that this is a mod I might have confused it for a real spinoff. The trailer is an Source Filmmaker-powered treat, showing off some new guns as well as zombified versions of each of TF2's iconic mercenaries as cartoony as they are macabre. It doesn't share any gameplay footage, just a few screenshots, but the feature list is impressive: co-op with up to eight players, unique perks, and a barricade mechanic, and endlessly scaling waves of increasingly intense difficulty.…