Make cell blanking work as expected in structured grids

This change makes it so that the default rendering path will
properly hide blanked cells. When surface filter is given
a structured grid with blanking, it now takes the slow
generic path because blanking will expose interior faces.
In there, it now skips blanked cells and treats faces of
cells that are shared with blanked cells as exterior faces.

Change-Id: I23d6397f8f9c7bbda3c366d160eaa03c4668be48
Merge topic 'structgrid-blanking-fixes' into master

891dd74e Make cell blanking work as expected in structured grids