replaced Mod_LoadSkinFrame functions with R_SkinFrame system, this

replaced Mod_LoadSkinFrame functions with R_SkinFrame system, this
allows multiple models to share textures in a given level, as well as
models using the same texture multiple times (often the case in md3
where meshes have a vertex limit, and q3bsp where texture structs
describe surface properties and multiple may exist referring to the same
texture set)
this reduced Nexuiz video memory usage by over 150MB

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7168 d7cf8633-e32d-0410-b094-e92efae38249