No longer used tiled bitonalization for foldouts, which was causing
problems when a small piece of paper was imaged against a large, black
background with lots of glare.
For an example, see testimg_foldout/manfredlewandowsb001f013_raw_0002.JPG
Make the black_pixel_percentage a parameter you can pass in to the
autocrop_remove_bg functions, and set it to 0.95 (instead of 0.90)
for foldouts. This allows autocrop to detect the outside edges of
folios more accurately. See items kempnerfamilyb013f001_0001 and
manfredlewandowsb001f013 for examples.