layout: Paint stacking contexts' overflow areas properly.

This was making `box-shadow` not show up in many cases, in particular,
but the effects were not limited to that.
auto merge of #4460 : pcwalton/servo/stacking-context-overflow, r=glennw

This was making `box-shadow` not show up in many cases, in particular,
but the effects were not limited to that.

r? @glennw