support for %s_%i.skin files on alias models (I.E. progs/test.mdl_0.skin), which...

support for %s_%i.skin files on alias models (I.E. progs/test.mdl_0.skin), which contain "replace" commands, followed by the name of the mesh skin to replace ("default" for mdl/md2), and the replacement name, any skins that are not replaced are set to not draw (this means of course that a model can have extra pieces that become invisible in some skins)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@3325 d7cf8633-e32d-0410-b094-e92efae38249