So far, the textures that we've applied to objects have been mostly static - they don't change, except if we change the texture coordinates to move them.
In some cases, we would like our textures to change.
Since a texture is basically a single image, we can imagine using a series of images, like a flipbook, to create an animated texture.
This can be done by creating an array of textures, from a sequence of images, and changing which texture is applied to our object over time.
Example: anim-planet.cpp