Show sketch vertex for all geometry types

The Draw interaction always shows the sketch vertex for points, but for lines and polygon only after the first vertex was set. This pull request changes behaviour so the sketch vertex is showed for all geometry types before the first vertex is set. See https://groups.google.com/d/msgid/ol3-dev/2500aceb-cb9c-4407-81ca-5280698cc7d0%40googlegroups.com.