some cleanup of shader permutation handling

some cleanup of shader permutation handling
expanded shader permutation system to allow multiple shader source files (not sure if this will ever be used) rather than just "glsl/default.glsl"
added code support for geometry shaders (not actually hooked up because there's no GL_ARB_geometry_shader extension yet)
added flagging of which shader objects are needed by a given permutation (to allow vertex-only shaders for shadow volume projection and such, also not implemented)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6749 d7cf8633-e32d-0410-b094-e92efae38249