`alphaGen portal` keyword is not implemented yet

[illwieckz]

This is a regression compared to Tremulous, but the Dæmon engine GL2+ renderer never did it yet, so it's a feature request.

The lack of support for alphaGen portal is technically not a bug to fix, it's a feature request: the feature has to be implemented or to be finished, which is not the same amount of work than fixing something that worked before, one cannot make it work by bisecting or something, the code is not there yet.

A map like nova is affected, viewpos 9633 3373 -21 56 -3, the portal should blend with a texture with distance.