fdo#75487 Store svgz decompressed (as plain svg) in the document.

Put decompressed svg in GfxLink so that when the document is
saved, the graphics is stored as plain svg. This is done so
because of backwards compatibility (older LO won't be able to
read compressed svg data in the document) and because the document
itself is already compressed.

Change-Id: I125005dfba785701e756487092f25bb90bcd2bcb