Dark Overlay Fixes

[andrikpowell]

Sorry Re-did this PR with a different branch name.

I'm kinda treating this PR as my Dark Overlay omnibus PR.

[andrikpowell]

Let me know if you have a better idea for the Hexen fix (Software is what has the issue).
Problem is that the black colour is taken from colormaps[0] and that gets replaced when it swaps the colormap with the "FOGMAP" during the maps with fog.

There are two ways to approach this:

1. Current way is setup to check for "FOGMAP" specifically, and use the main colormap if true
2. Instead of checking for "FOGMAP", we can just check if the fadetable is set to anything other than the main "COLORMAP"

Edit: I actually think the second option is better, since it's same way that dsda_HexenApplyFadeTable() is done

[Pedro-Beirao]

I dont know how common it is in the code, but I dont like seeing W_GetNumForName being called every frame.

Could you use the variable LevelUseFullBright? It is set to false if FOGMAP is being used. No need for that new function

[andrikpowell]

I dont know how common it is in the code, but I dont like seeing W_GetNumForName being called every frame.

Could you use the variable LevelUseFullBright? It is set to false if FOGMAP is being used. No need for that new function

Ok, here's a completely different approach. I went with your LevelUseFullBright idea, but I also addressed running the "COLORMAP" lookup on every frame as well.

Curious what you think :)