It's a bit unusual for a game that's graphics based somewhere between 1998 and 2000 but Phantom Fury which looks like Half-Life 1.5 does it and it looks very crisp. Isn't it just a GL_NEAREST instead of GL_LINEAR when creating a texture? Actually you should combine it with glShadeModel(GL_FLAT) otherwise it looks really odd... but that might destroy the lighting on the walls, ok I see it's not so easy. So, please add that option in "Zortch 2" 😉