Todo: Add a n-dimensional browsing canvas which follows diagonal
	directions, to accelerate erosion on diagonal lines.

	* mln/morpho/erosion.spe.hh:
	* tests/morpho/erosion.cc: More tests and benches on rectangles,
	hline2d, vline2d and octogone.

	* mln/canvas/browsing/directional.hh: Improve this canvas in order to
	use it in erosion, to improve performance on erosion with lines.

	* tests/timer.hh: New. Tool to bench tests.

git-svn-id: https://svn.lrde.epita.fr/svn/oln/branches/cleanup-2008@2532 4aad255d-cdde-0310-9447-f3009e2ae8c0