Remove TEXTURES name length limit

As discussed on the zdoom forum:

http://forum.zdoom.org/viewtopic.php?f=15&t=45887

These changes make it so that texture names are now "unlimited" in length.

I've only verified that these changes work as expected for sidedefs in a TEXTMAP when referencing textures defined in textures.txt.

I'm creating this pull request so that there's a record of the precise changesets involved as I plan on making additional changes (where needed) to support the use of "long" texture names in other places such as FONTDEFS, SBARINFO, DECORATE, etc.

I *do not* plan on changing sprites for reasons already discussed in another thread:

http://forum.zdoom.org/viewtopic.php?f=15&t=45824