Many dvd's are showing up that have titles that have numerous repeated
language unit tables and pgc's. The new release "Up" is an example.  I
believe "Dark Knight" also exhibits this. When scanning such discs
with lsdvd (and other programs that scan all titles), libdvdread will
consume many GB of data for these repeated elements and gets very slow
(e.g hours to read all titles) on OS X due to reading the duplicate
data from uncached raw devices.

This patch detects duplicates and reference counts pgc's and pgcit's.
When a duplicate is detected, a reference count is incremented instead
of allocating new memory and re-reading the data.