GDIMetaFile::dumpAsXml() is very useful while investigating/examining
the meta files, but it overwrites with the same file name each time.
To be able to get different dumps at the same time, let's add
a fileName parameter to it.

Change-Id: I993720b460f326cd65519556cf1e902591d90d42
Reviewed-on: https://gerrit.libreoffice.org/66283
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>