Front/back seems like the wrong terminology when discussing
actors arranged in layers. Top/bottom fits better with Clutter
API function names and other recipes, so renamed variables.
Cross-fading between two images is straightforward,
but cycling between more than two is more efficient
if done by copying COGL textures between the
two textures, rather than trying to reposition the
textures.
The example demonstrates how to reuse a pair of
textures to cycle through multiple images.
