As your model of Mac only has an Intel video system, that is probably the cause of your problem - that has not been supported hardware since game release. All you can do is try to set the renderer to openGL in our launcher-Game Settings menu.
If that doesn't help, all you can do is revert to the previous game version in the Steam betas tab for the game. Or play on any other machine you have access to with supported hardware. OR if you have Apple Bootcamp installed, play in Windows there.
Sorry about that!