Allow use_cropbox to be controlled

[donaldgray]

Issue raised where rasterized image doesn't reflect what is shown when looking at the origin PDF through a standard viewer.

This appears to be due to pdf2image library using the media_box by default, where crop_box seems more appropriate.

From online article on PDFs:

The MediaBox is the largest page box in a PDF. The other page boxes can equal the size of the MediaBox but they cannot be larger.
The CropBox defines the region to which the page contents are to be clipped. Acrobat uses this size for screen display and printing.

use_cropbox is an available pdf2image option but it defaults to False (so uses mediabox). This change introduces a new envvar, PDF_RASTERIZER_USE_CROPBOX, to control this, maintaining default to avoid any unintented changes in behaviour.