* core/init_integral_image.hh: New.

	* src/binarization/sauvola_ms.cc: Optimize and make it more
	robust.

	* canvas/integral_browsing.hh: New canvas to browse and compute
	data in an integral image.

	* binarization/internal/first_pass_functor.hh: New functor to be
	used in the integral browsing.

	* binarization/sauvola_threshold.hh: Add new overloads.

	* subsampling/integral_single_image.hh: Subsample an image and
	compute integral images at the same time.