Currently in case of RTF export LO converts all unknown graphic into WMF. And some images due to zero prefSize
produce invalid WMF of zero dimensions.

As a workaround for current case we do not convert GIF images to WMF, PNG format looks better here. The code can
be extended to provide suitable converters for other formats.

Change-Id: I0c98111a4496250874aa4f664dd2f70134c1bdd1
Reviewed-on: https://gerrit.libreoffice.org/19581Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>