GL still has its own functions for enabling and disabling fog while
rendering, but GLSL doesn't need such (thanks to the shaders), nor will
vulkan (and the software renderers don't support fog).
The static variable meant that Fog_GetColor was not thread-safe (though
multiple calls in the one thread look to be ok for now). However, this
change takes it one step closer to being more generally usable.
Patch found in an old stash.
