Fix glitches with flats visible when looking up/down in 1st person (software renderer fix)

Hope you don't mind Jimita, but we really would like to have this fix for software at least, so I made a copy of it on our repo a while back.

Fixes exactly what it says in the title.

merged

mentioned in commit 5069acb1

Owing to the fact Jimita never made a MR for this (and appears to have vanished for now), I've gone ahead and merged this in now.