r_showcollisionbrushes additions and new r_celtexturing render mode

- fix bug with r_showcollisionbrushes draw all submodels as worldmodel.
- new cvar r_showcollisionbrushes_filter (filters collisionbrushes by
type: all, BIH brushes, BIH collision triangles, BIH render triangles)
- new cvar r_celtexturing, r_celtexturing_brightness, r_celtexturing_mix
to enable 'cel texturing mode' which simple forces average-color for
diffuse RGB (alpha and all other layers are keeped as is). GL 2.0 only.
And DDS-only for now (and only RwgTex generated ones) because it is
implemented for only DDS-stored avgcolor.