Always add first segment when appending coordinates

When we have only a single point, we want to make sure it gets added (fixes #1821).

Because this code doesn't differentiate coordinates that are part of a linestring or linearring from those that are independent points, we always add the first segment (or pair of points).  In addition, coordinates for segments are appended whenever they represent a change in relationship with respect to the extent.  This keeps the code simpler for handling fills and properly capturing intersection points for linestrings.  This could be modified to save a few extra coordinates, but it provides for simpler code at a very minimal cost.
Merge pull request #1822 from tschaub/append-first

Always add first segment when appending coordinates.