Update to 3.3.7

- Bug fixes / improvements
    - Fix abort in RightmostEdgeFinder (#605)
    - Do not force precision reduction below 6 significant digits
      while trying to obtain a valid Buffer output (#605)
    - Fix GEOSPointOnSurface with zero-length linestring (#609)
    - Fix EMPTY return from zero-area polygon (#613)
    - Segfault from symdifference (#615)

(gdt)