Fix portal visualisation

[hackgrid]

Originally portals used 8 polygons prior to 8b216e7, after that 16.

Currently the portals looked a little bit frayed, because the "hole cutting in the walls" still uses 8 polygons, but the visualisation 16.
In the old repo we also had issues with details about that, I don't know if these are still accessable via archive.org.

He also mentioned this problem in his video here (from 5:00 on)
https://youtu.be/sb3nHlsBBpg?si=0SmWbb7D8U30vF-G&t=301

[mwpenny]

Thanks for logging this so it's tracked. After a cursory look on archive.org I couldn't find the issue in the original repo. At some point I'll go through in more detail and record any archived issues missing from here (I also have some in a text file I need to do the same with).

Regardless of whether it was archived, I'll be going through the portal placement / hole cutting code in the future to understand it, potentially simplify it, and make changes like these.