- fix SEGFAULT when a .shader file has more layers than allowed

- fix SEGFAULT when a .shader file has more layers than allowed
- reduce number of layers from 8 to 2 to save memory (increase it later when additional layers are actually USED)
- parse ALL layers, even those behind the limit, to check for lightmap references and similar stuff
- when looking up a shader, only look through the actually DEFINED layers (not through all ALLOCATED ones), this means that the OS won't allocate pages more shaders than actually used
- increase maximum number of tcmods from 4 to 8 for Nexuiz

net: 1300MB -> 900MB vsize, and less RSS used too; so it won't swap any more on a 1GB system

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7630 d7cf8633-e32d-0410-b094-e92efae38249