routine. Note that it is not the "real" sharpen which should be built
on top of a gaussian kernel.
git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@2662 92316355-f0b4-4df1-b90c-862c8a59935f
about twice as fast as the original code in research/2008-displacement.
git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@2644 92316355-f0b4-4df1-b90c-862c8a59935f
sam