Add an option to fill the whole page with a picture

[mportesdev]

This would be useful e.g. for creating PDFs from scanned documents where the picture aspect ratios are very close to (but not exactly the same as) the page aspect ratio.

[mportesdev]

This would introduce a fill_page option with the default value of False.

When set to True, this option would conflict with the following options:

• layout --- the layout would be forced to 1x1
• margin --- the margin would be forced to zero
• stretch_small --- the picture dimensions would be forced to the page dimensions

[mportesdev]

Alternatively, introduce a fill_area option that would not conflict with layout and margin. Filling the whole page with a picture would then be achieved by setting margin=0 and fill_area=True (using the default 1x1 layout).