Merge branch 'other-software-fixes' into 'master'

Some other software fixes/changes

Some other software fixes/changes I've made

* Change 1: single-sided linedef midtextures (read: not double-sided linedefs, but single-sided) now don't skew if Effect 2 is checked. Before, there wasn't any ability for software to disable skewing for them. =V
* Change 2: PolyObject back side midtextures now return to being where they should be. Had to make a hack to fix this though.

See merge request !86