Remove duplicate contexts on stacking context stack
Remove duplicate contexts on stacking context stack

During display list flattening stacking contexts were being push onto
the stacking context stack twice, due to a merge issue. It seems that
two PRs added this functionality, so it was duplicated. This PR removes
the duplicate entries (which were mainly harmless) and cleans up the
code slightly.

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