Respect unknown point counts when writing points

Add support for writing when a source stage has an unknown
number of points. New features:

* Update Stage so, if it reports 0 points for getNumPoints, it inherits the
  point count type from the previous stage. This allows propagation of a
  PointCount_Unknown type all the way from the source stage.

* Update the base writer to respect a PointCount_Unknown.

* Tweak the faux reader slightly to not force a PointCount_Fixed
  type, and give it a new constructor which can be used to initialize
  the point count type.