Refactor DynamicScene vertex positions

1. Remove `DynamicObject.vertexPositions`
2. Add equivalent `positions` property to `DynamicPolygon`, `DynamicPolyline`, and `DynamicWall`.

This goes along with AnalyticalGraphicsInc/czml-writer#70

I was hoping to include support for polygon holes in this update, but that will now happen in a future pull.
`entity` is not really an Entity anymore, is it?  It's the `PolygonGraphics` (etc.) object.