[gl,glsl] Copy fog color rather than return pointer

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.

include/r_internal.h

@@ -31,7 +31,7 @@ void Fog_Update (float density, float red, float green, float blue,
 struct plitem_s;
 void Fog_ParseWorldspawn (struct plitem_s *worldspawn);
 
-float *Fog_GetColor (void);
+void Fog_GetColor (quat_t fogcolor);
 float Fog_GetDensity (void) __attribute__((pure));
 void Fog_SetupFrame (void);
 void Fog_EnableGFog (void);