changed cl_stainmaps to be a loadtime cvar which decides whether to

changed cl_stainmaps to be a loadtime cvar which decides whether to
allocate stainmaps at all, this saves some memory if it's 0 (which is
the default)
changed stainmap allocation to lump all allocations into one big one,
this saves 1.5MB of memory system overhead (tracking about 28000
allocations for stainmaps in cavetest2.bsp has a lot of overhead) and
probably a good deal more malloc overhead on top of that

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8258 d7cf8633-e32d-0410-b094-e92efae38249