Negative A_Light Inverts Colormap

[NeuralStunner]

In ZDoom, I can give negative values to [wiki]A_Light[/wiki] to darken visibility. In GZDoom, attemtping this results in the inverse colormap being applied until the light level is restored to >= 0. It would be more desirable to have the darkening effect, consistent with ZDoom. (And effectively make this a documented feature of A_Light, as well.)

[Graf Zahl]

fixed