When SurfaceCache::Lookup is called to access surface data, it indicates
that the caller will not accept substitutes as in the case of
SurfaceCache::LookupBestMatch. As such, we need to be careful not to
remove those surfaces from our cache when pruning (in part 8b). This is
the marker used to track that, at some point, there was a caller which
got this surface that would accept no other (e.g. factor of 2 mode must
make an accept for this particular surface).
Andrew Osmond
Seth Fowler