Convert shapes on the document to black & opaque
before sending it to the pdfexport filter.

Convert them back to gray & transparent after
the export operation.

Change-Id: Iffe66e371710f16ef1c04f0da196fc5a561af344
Reviewed-on: https://gerrit.libreoffice.org/67517
Tested-by: Jenkins
Reviewed-by: Muhammet Kara <muhammet.kara@collabora.com>