Fix intersection plane / cone when cone has negative altitude

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28397 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
4.9.199.0

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28398 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e