when modifying R-tree indices we forgot to unpin pages (causing
flush at-end-of-statement to fail - because page is pinned - and thus
mark table corrupted) and also to unlock key_del (causing assertion
failure info->used_key_del==0 at maria_close()).


storage/maria/ma_rt_index.c:
  When modifying pages of R-tree indices we keep them pinned, and
  lock key_del, so we here make sure to unpin and unlock at the end
  of the modification. I checked all functions which add to info->pinned_pages
  or call _ma_lock_key_del(): the parent is always maria_rtree_insert()
  or maria_rtree_delete(), which thus do the unpin/unlock now.
mysql-test/r/maria-gis-rtree.result:
  new result, which is incorrect (filed as BUG#35351) but at least
  test does not crash anymore
mysql-test/t/maria-gis-rtree.test:
  test of Maria's GIS functionality (R-tree indices etc); copied from
  MyISAM's gis-rtree.test; used to crash without the bugfixes.