Feature request: set dpi for pdf.py via environment variable

[groovy9]

I've been using jbig2enc for years to make PDFs of monochrome scans (thanks for that!). But every time I install it, I have to remember to edit pdf.py and set dpi = 300 to match the dpi setting of my scanner script.

It'd be a useful and trivial addition to have pdf.py look for an environment variable like JBIG2ENC_DPI and default to 72 if not found. That way, I could set it in my scanner script and forget about modifying pdf.py.