ds-models (3D model packs for gzdoom and skulltag)

ds-models allows playing Doom, Heretic, and Hexen with OpenGL and 3D
models, which gives it a much more modern look.

This model set is released by -=Dark-Assassin=- on the
SkullTag forum. For more information, see the forum thread at
http://www.skulltag.com/forum/viewtopic.php?f=13&t=24238

If you never plan to use models with skulltag, passing SKULLTAG=no to
the build script will omit the skulltag-specific models. The resulting
package with be approximately 30% smaller. There's also a GZDOOM=no
option, but it won't make the package smaller (skulltag still requires
the gzdoom models, so they'll still be included).

As of r15, ds-models supports Zandronum, but there's no SlackBuild for
Zandronum yet, so no support for it here (yet).

This includes scripts "ds-gzdoom" and "ds-skulltag" for starting gzdoom
or skulltag with the models preloaded.

To make gzdoom or skulltag always use the models, edit
~/.config/gzdoom/zdoom.ini and/or ~/.skulltag/skulltag.ini and add the
following lines under the [Global.Autoload] marker:

# for both gzdoom and skulltag:
Path=/usr/share/games/doom/GZ-Models.pk3

# for skulltag only, also add:
Path=/usr/share/games/doom/ST-Models.pk3

Note that the models will only be displayed if gzdoom/skulltag is using
the OpenGL renderer (Options | Display Options in the in-game menu).
