I’ve decided not to implement the gamma fade.
In a real gamma fade you could set the gamma ramp to a particular value,
and then blit normally, with all colors being modified by the gamma ramp.
This is a cool way to implement “bloodlust haze”, or dimmed lights.
Unfortunately, this cannot be generally emulated with any degree of speed.
You’re better off building your own optimized color lookup tables for
these effects when gamma fading isn’t available.
The nice thing about real gamma ramp modification is that you don’t get
any speed loss for these effects – they’re free!