[Binary] Ceiling verx slopes (same tag) do not work

I don't know how this got missed, but it did. Ceiling vertex slopes that use the same tag don't work anymore. So far this seems to be a weird oversight, but it can be seen when using the example wad file provided by the wiki https://wiki.srb2.org/wiki/File:Ex_ld715_slopebacksideceilingbyvertices.wad

In game

In Editor

Vertex ceiling slopes that use different tags seem to work though.