Portalsky code a la Half Life 2 (with parallax change)

I've implemented portal skies like the ones in Half Life 2 (with changing parallax) this could be useful for all mod developers, I request a pull, so everyone can use it freely

the original code is from d3xp by Nerve software
Ivan_the_B and Neurological implemented it back to doom3 game code
and I modded it heavily in order to get the Half Life 2 like portal skies

this mod needs new assets for the portal skies to work, all those assets can be found in my previous implementation of this technique for iodoom3 here:
https://github.com/BielBdeLuna/portalSky