[Feature] GLDefs "FilterLights" flags for brightmaps

[MajorCooke]

I've discovered that subtractive lights can override brightmaps, which seems to be a bug, but at the same time it's a cool effect that could prove useful, so I actually consider it a feature.

But there are definitely times where brightmaps being overridden by GL lights is unwanted. I was trying to create a special texture with some black lights, hoping to keep certain parts glowing, but unfortunately it's impossible to achieve as it stands.

If there could be a flag for GLDefs to force brightmaps to take priority (while also respecting translations), that'd remedy my issue.

While I'm not expecting this to be possible, an added bonus would be to add separate flags as such:

• FilterLights - The stronger the brightmap, the less it factors in dynamic lights
• FilterSectors - ^ but for sector colors of all kinds.
• FilterEffects - Applies both of the above.

But if it must be all or nothing, that'll work.