St: Fix fade effect

Commit 4095a58eb9a2921c178b30c56d1fbe7662c2778a introduced a
regression, since we have to take into account four cases,
top, bottom, right, and left, and that can't be merged into
two like that commit did.
So fix it to make fade effect works again.

https://bugzilla.gnome.org/show_bug.cgi?id=708256