The three biggest genres in gaming were invented by people working for free. Counter-Strike started as a Half-Life mod that two students duct-taped together in 1999. Valve hired them, and the franchise still pulls in close to a billion dollars a year from weapon cases alone. Dota was a custom Warcraft 3 map. One map. It spawned League of Legends and Dota 2, which have been printing billions for Riot and Valve every year since. Battle royale traces back to Brendan Greene, a photographer with…