optimized surface rendering to surface->groupmesh->data_* array pointers directly...

optimized surface rendering to surface->groupmesh->data_* array pointers directly instead of indexing them with surface->num_firstvertex, this allows fewer pointer changes (a small speed gain)
fixed stupid bug in q1bsp texture loading which was making all missing textures invisible
changed portal generation to not break when a portal is clipped away, this fixes a crash at render time with bogus portals
changed r_drawportals to only draw portals surrounding the visible leafs (speed gain, but also useful for debugging vis glitches)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@5153 d7cf8633-e32d-0410-b094-e92efae38249