Add JPEG compression of watermarked PDF pages

While comparing watermark.sh to the Python version, I found out that the PDFs after watermarking have significantly bigger file sizes when using the Python script (watermarking 50 PDFs with each 16 pages, the Bash script 81MB and the Python version 180MB). In the Bash script, we allowed for both rastering PDF pages to images with an DPI-parameter and saving the watermarked images back as pdf with both DPI and JPEG quality parameter. I suggest to give the user the opportunity to enable JPEG compression here as well.