When including a PDF as an image, it's represented internally as
a Bitmap with additional PDF data. On SwapIn, LibreOffice just
imported the PDF data missing the PDF preview. The Graphic also
gad the wrong image type, which results in a busy loop on master,
with a strange / unhelpful STR_COMCORE_READERROR generated by
SwNoTextFrame::PaintPicture.

This is a workaround to generate the Bitmap on SwapIn, which
will really slow down LibreOffice when importing many PDFs.

I guess the job of generating the PDF previews should probably
be deferred to a thread or a low priority Scheduler task, just
like the general image loading is handled.

Change-Id: I8084e4533995ecddc5b03ef19cb0c6a2dbf60ebd
Reviewed-on: https://gerrit.libreoffice.org/43906Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>