Q2Pack support and small Q2W enhancements

33eff58 adds a small fallback code path when loading models: if no shader could be loaded (e.g malformed model, or model does not provide shader path), we try "skin.tga" in the model's directory. This fixes all of the Quake2World models in Radiant, and it shouldn't hurt any other game.

8333330 cleans up the libGL resolution in `main()` so that we're only supporting `GDK_GL_LIBGL_PATH`.

The rest of this is to get Q2Pack working, which it does now.