force saturation to be in [0,1]

git-svn-id: svn+ssh://rubyforge.org/var/svn/tioga/trunk/tioga@622 f6f669a6-2b79-499e-8f05-be2b991fc3d0
[tioga] add test for (problematic) RGB/HLS conversion

git-svn-id: svn+ssh://rubyforge.org/var/svn/tioga/trunk/tioga@623 f6f669a6-2b79-499e-8f05-be2b991fc3d0