* clutter/clutter-behaviour-depth.c: Clarify that what drives
the movement along the Z axis is the ClutterAlpha object (we
don't have the luxury of a rollover like the opacity does);
so, if you want to go from 0 to -100 you have to use a
decreasing function, just as well if you want to go from 100
to 0. Using a min-depth of 100 and a max-depth of 0 and an
increasing function is undefined behaviour.
* tests/Makefile.am:
* tests/test-depth.c: Add a test case for the depth behaviour.
