test/alphamap.c: Make dst and orig_dst more independent of each other

test/alphamap.c: Make dst and orig_dst more independent of each other

When making the copy of the destination, do so separately for the
image and the alpha map. This ensures that the alpha channel of the
alpha map will be different from the alpha channel of the actual
image.

Previously, orig_dst would be copied onto dst along with its alpha
map, which mean that the alpha map of orig_dst would become the new
alpha channel of *both* dst and dst's alpha map. This meant that test
didn't actually test that the alpha maps alpha channel was actually
fetched.