A set of changes improving our RTree indexes and fixed few bugs
found during the tests


myisam/rt_index.c:
  Algorythm for picking the branch to insert was fixed.
  pick_by_perimeter version of the algorythm added (mostly for testing
  purposes)
myisam/rt_index.h:
  minimal size of the page set to 1/3
  It noticeable increases searching performance
myisam/rt_key.c:
  counting of the size of the filled part of the page fixed
  rtree_choose_key moved to rt_index.c
myisam/rt_key.h:
  no need to make rtree_choose_key global
myisam/rt_mbr.c:
  operations for counting the perimeter of MBR added
myisam/rt_mbr.h:
  interface for rtree_perimeter_increase
myisam/rt_split.c:
  my_multi_malloc changed with my_alloca
sql/spatial.cc:
  LINESTRING object can consist of single point