Fix for bug #848901 Assertion `fabs(cur_isc->x-m_cur_intersection->x) + fabs(cur_isc->y-m_cur_intersection->y) < 0.000000000001' failed in Gcalc_scan_iterator::intersection_scan() in maria-5.3-gis

        That assertion's check was too tight. Released it a bit.

per-file comments:
  mysql-test/r/gis-precise.result
        Fix for bug #848901
        test result updated.
  mysql-test/t/gis-precise.test
        Fix for bug #848901
        test case added.
  sql/gcalc_slicescan.cc
        Fix for bug #848901
        The DBUG_ASSERT check is too tight here.