better check for convexity

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31972 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
GeoPolygon says if it's a part of GeoPolyhedron

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31973 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e