PPT: Improved access to image names.

* It seems to be normal for MS Office 2003 that the MD4 digest of a BLIP
(specifies the unique identifier of the pixel data in the BLIP) in blipStore
and OfficeArtDggContainer DOES NOT match. This means that we can't locate
the picture for a Picture Shape in the blipStore b y its MD4 digest.

* Using stream offsets to locate the picture.

* Re-save in MS Office 2007 updates the MD4 digests in OfficeArtDggContainer.
Tests showed that MD4 digests in blipStore were always up2date.