We use get_geometry_type() call to decide the exact type
of a geometry field to be created (POINT, POLYGON etc)
Though this function was only implemented for few items.
In the bug's case we need to call this function for the
Item_sum instance, where it was not implemented, what is
the reason of the crash.
Fixed by implementing virtual Item::get_geometry_type(),
so it can be called for any Item.


sql/item.h:
  Bug #31155 gis types in union'd select cause crash.
  virtual Item::geometry_type() added instead of
  various geometry_type() fucntions.
sql/item.cc:
  Bug #31155 gis types in union'd select cause crash.
  
  Unified virtual ::get_geometry_type() function used
sql/item_geofunc.cc:
  Bug #31155 gis types in union'd select cause crash.
  
  virtual Item::geometry_type() implemented for geo-Items.
  Mostly previous ::get_geometry_type() implementation changed
sql/item_geofunc.h:
  Bug #31155 gis types in union'd select cause crash.
  
  get_geometry_type() declarations unified