This is an optimization to avoid painting backgrounds
that are obscured. It's a lot of complexity that it's
not clear we'll need given that we're using a GL backend.
Also, we can add it back in more easily/efficiently in the future
once we have a display list architecture.
This also means we can remove the needsPaintInvalidation
dirty bit and some opacity information on filters.
R=esprehn@chromium.org
Review URL: https://codereview.chromium.org/856563006
4256d4a224