No need to convert to PDF with LibreOffice, just convert to PNG

Exporting all pages of a document to a PDF is a waste of time. All we
need is a thumbnail of the first page anyway. Plus, reading that PDF
(even just the first page of it) into imagick is presumably much
slower than reading a simple PNG.

Signed-off-by: Tor Lillqvist <tml@collabora.com>