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.
Modified the code example for the Clutter API version
of the cross-fade to use the same command line
as the COGL version.
This also simplifies the explanation in the recipe.
Also made the COGL code sample more consistent with
the Clutter API code sample.
Added simple image viewer which loads image file names
from a directory, displays the first one, then displays
the next in the list with each key press. Uses the
primitive fade front in/fade back out approach.
Also adapted Emmanuele's example code which uses Cogl
to produce a similar effect, but within a single texture.
This code loads two images specified on the command
line and cross-fades between them.
