Destructible geometry: GZDoom part

[jewalky]

This is a complex PR. Details can be found here: https://forum.zdoom.org/viewtopic.php?f=59&t=62420

[ZippeyKeys12]

Woah, interesting idea. Portals are hell on earth.

[coelckers]

Copied from the forum post:

Can you please reorganize this that it isn't scattered throughout the code? This is such a complex and extensive features that all related code should be in its own file so that they are easier to find later. Also, after doing that, please squash the commits into one.

This looks like something that will most likely be extended later, and nobody wants to hunt for all the code involved.

[coelckers]

I merged this into a local work branch for evaluation. Closed so that nobody merges it while I'm working on this code.