Add test case for #633 based on GEOSOffsetCurve from C-API. Passing negative distance for right-sided offset curve generates correct/expected output. Interestingly, equivalent test using BufferBuilder directly, from C++ API, does not pass.

git-svn-id: http://svn.osgeo.org/geos/trunk@3819 5242fede-7e19-0410-aef8-94bd7d2200fb
Add comment missing from previous commit.

git-svn-id: http://svn.osgeo.org/geos/trunk@3820 5242fede-7e19-0410-aef8-94bd7d2200fb