[giac] further fixes

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24497 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] changes to support default variable != x

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24498 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Better rendering for spheres

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24499 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e