1. Threat MBR for a key as double[] and convert it only
when about to store it on disk.
2. Remove the redundant function get_double().


myisam/sp_key.c:
  Bug #29070: 
  1. threat MBR for a key as double[] and convert it only
  when about to store it on disk.
  2. remove the redundant function get_double()
mysql-test/r/gis-rtree.result:
  Bug #29070: test case
mysql-test/t/gis-rtree.test:
  Bug #29070: test case