This BD style magic wall converts rocks into diamonds and vice versa. Note that
a magic wall can only be activated for some limited time. It can also turn mega
rocks into nitro packs, nitro packs into mega rocks. Even flying diamonds and
rocks pass them to be converted into each other, but they do that from bottom to
up, of course.

The duration in seconds for which magic walls are active is configurable.
A duration of zero will let the wall run forever.
All magic walls share the same timer.