textures: Add missing Plutonia texture definitions.

The Plutonia WAD includes some extra textures (and associated patches)
compared to the original doom2.wad. Add these missing textures, with
dummy "Plutonia versions" of textures where they conflict with
existing ones, so that we include all patches needed for compatibility.
Fixes #42.
textures: Add textures.cfg as make dependency.

If textures.cfg changes, rebuild all texture lumps.