Fix polyobject midtextures being stretched into infinity

Regression introduced by !2329 (merged), cause is similar to !2533 (merged). Basically, when trying to fix the texture scaling, it was changed to use not only the texture scale of the target sector (which is correct), but also the entire polygon itself, causing it to stretch through the entire sector instead of just the polyobject. This patch reverts some of those incorrect changes while keeping the correct ones, thus fixing both problems.

This bug can be observed on Disco Town, where the driving cars on the road near spawn has it's textures stretched infinitely into the sky.

Possibly fixes #1302 (closed)