What kind of films are Latin America and Spain’s new generation filmmakers making?  One answer will be supplied in May at the Cannes Film Festival which includes a showcase, Marché du Film ’s Ventana Sur Goes to Cannes – Río de la Plata, highlighting five first features. Among them are Uruguay’s buzzy dramedy “When I Existed,” Spain’s supernatural thriller “The Mantises” and “The Mark of the Jaguar,” a Brazil-Mexico animated Aztec action adventure. The showcase’s five titles, all in post-