When reading a DCP set up asset hashes from the CPL/PKL, not by digesting the actual...

When reading a DCP set up asset hashes from the CPL/PKL, not by digesting the actual asset.

Previously when reading a DCP we would recalculate asset hashes.  This meant that corrupted
DCPs could be loaded in and their hashes re-written (if nobody checked the new hashes against
the ones in the PKL/CPL).

It seems better to take the hashes from the metadata.  Then if the assets are corrupted
and the DCP is re-written the PKL/CPL hashes will be preserved, showing rather than hiding
the corruption.