A. Dickenstein was partially supported by UBACYT 20020100100242, CONICET PIP 20110100580, and ANPCyT PICT 2013-1110, Argentina.
We are indebted to Atsushi Ito for fruitful discussions and for allowing us to quote his counterexample 3. We thank the referees for their detailed and useful comments to improve our manuscript. Almost 30 years ago, at an NSF Geometry Institute held at Amherst College, Bernd Sturmfels introduced us to many of the topics discussed here. We are grateful for his inspiration and his friendship throughout this time.