Support glTexGen

[pmiddend]

With glTexGen, you can let OpenGL generate texture coordinates for you. This is useful if the coordinates aren't static, but depend on the viewer, for example. Two use-cases:

• Reflection mapping with cube maps
• Shadow mapping

For shadow mapping, you probably need some other extensions, so that would involve more work. Reflection mapping would be nice, but you can easily (portably) do this with shaders.