Add ST_ClipByBox2D and lwgeom_clip_by_rect (#2939)

Includes testcases and documentation
Requires GEOS-3.5.0+

git-svn-id: http://svn.osgeo.org/postgis/trunk@12999 b70326c6-7e19-0410-871a-916f4a2858ee
Fix regression suite to skip ST_ClipByBox2D testing unless available

.. it was a typo (requires GEOS 3.5, not 3.4)

git-svn-id: http://svn.osgeo.org/postgis/trunk@13000 b70326c6-7e19-0410-871a-916f4a2858ee