aperio: Record missing tiles during open
aperio: Paint missing tiles by downsampling from previous level
aperio: Rerender tiles concatenated from missing tiles

The Aperio encoder appears to get confused when generating downsampled
tiles that include missing subtiles: the missing subtiles are rendered as
black squares, and the other subtiles are rendered in the wrong places.
Treat downsampled tiles incorporating a missing tile as though they are
also missing, and rerender them.

http://lists.andrew.cmu.edu/pipermail/openslide-users/2014-June/000806.html
aperio: Improve test coverage of missing-tile handling