Allow max fps to be automatically set to the screen refresh rate

[yani]

Right now we can set the max fps with FRAMES_PER_SECOND to the number we want, or 0 for unlimited. It would be nice to have some sort of AUTO option that would try and automatically set it to the refresh rate of the screen that's being used.

I'm quite sure you can get the correct refresh rate by querying the display mode in SDL2. https://wiki.libsdl.org/SDL2/SDL_GetDisplayMode

Another option is the SDL_RENDERER_PRESENTVSYNC render flag. Although I'm not sure if that actually limits the internal frame generation because afaik KeeperFX creates its own buffer. https://wiki.libsdl.org/SDL2/SDL_RendererFlags

I made the new Qt Launcher already set it during installation. But because we can't really know what screen the user wants the game to run on I went for the primary screen refresh rate. KeeperFX however would have a much easier time figuring out what screen it runs on.