Make text runs added inside a text-shadow stack also add visible text run.
Make text runs added inside a text-shadow stack also add visible text run.

The original implementation didn't quite match the expected semantics
of the new interface. Now, when a text run is added, add it to any
shadows on the stack, in addition to a normal visible text run.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender