Add an example that uses cube maps

[pmiddend]

The example could simply load a cube map texture that represents each cube face by a number. You can use cube maps without shaders, so this should be easy to pull off (we need 3D texture coordinates as indices to the cube map, though).

[pmiddend]

The example is finished, but should be improved to test render-to-texture with the cube faces, as well as Cg interaction with cube textures.

[pmiddend]

Render-to-texture is implemented in d61ad6a5f148fbee72f1cf8c77b8cfaf11fd43e3

[pmiddend]

Using Cg shaders is kind of blocked by #85