[giac] fix default var for limit

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24502 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] should improve PointList support

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24503 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e