some changes to make code nicer


include/myisampack.h:
  mi_sint1korr and similar things added to do conversion in an
  uniform way
myisam/rt_index.c:
  'if' simplified
myisam/rt_mbr.c:
  some fixes to make code nicer and smaller
myisam/rt_test.c:
  some modifications to extend test
mysql-test/r/gis-rtree.result:
  result became slightly different because of changes made