WKTWriter::appendCoordinate optimisation

Modified the WKTWriter::appendCoordinate method to not create an
intermediate std::string, but instead write directly into the Writer
object. The result is a small performance improvement.

Patch by Mats Taraldsvik <mats.taraldsvik@norkart.no>

git-svn-id: http://svn.osgeo.org/geos/trunk@3767 5242fede-7e19-0410-aef8-94bd7d2200fb