Now that the cards have rounded corners, draw the stack box behind the
cards with rounded corners as well. This way, the corner of the stack
box doesn't peek out from behind the cards.
The caveat here is that the "play" card stack now needs to hold a
reference to the "waste" stack beneath it so it knows when not to draw
its background on top of the waste stack. To faciliate that, the array
of card stacks is now a NonnullRefPtrVector so the play stack can store
a smart pointer to the waste stack (instead of a raw pointer or
reference).
