Multi-patch texture support for transparency AND translucency

Adds a column drawer to software renderer to support multi-patch textures for midtextures on two-sided linedefs with both transparency (i.e. invisible pixels) and translucency (set by linedef types 900-908). Before, you would have just gotten tutti-frutti or whatever it's called.

This was originally coded for 2.2, but I'm backporting it since I don't see any real reason to keep it back.