Replaced some model fields and changed their purpose slightly to simplify a lot of...

Replaced some model fields and changed their purpose slightly to simplify a lot of code:
model->firstmodelsurface : model->submodelsurfaces_start
model->nummodelsurfaces : replaced with model->submodelsurfaces_end
model->sortedmodelsurfaces : replaced with model->modelsurfaces_sorted (which starts at surface 0, not firstmodelsurface).

Changed the implementation of MakeSortedSurfaces so that it now sorts by effect, texture, lightmap, previously it didn't care about lightmap or effect.

No behavior changes.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@13072 d7cf8633-e32d-0410-b094-e92efae38249