Optimizator fails using index with ST_Within(g, constant_poly).

per-file comments:
  mysql-test/r/gis-rt-precise.result
        test result fixed.
  mysql-test/r/gis-rtree.result
        test result fixed.
  mysql-test/suite/maria/r/maria-gis-rtree-dynamic.result
        test result fixed.
  mysql-test/suite/maria/r/maria-gis-rtree-trans.result
        test result fixed.
  mysql-test/suite/maria/r/maria-gis-rtree.result
        test result fixed.
  storage/maria/ma_rt_index.c
        Use MBR_INTERSECT mode when optimizing the select WITH ST_Within.
  storage/myisam/rt_index.c
        Use MBR_INTERSECT mode when optimizing the select WITH ST_Within.