Move geometry specific code in Field_blob::get_key_image() to Field_geom

The code in field.cc

uint Field_blob::get_key_image(uchar *buff,uint length, imagetype type_arg)

{

#ifdef HAVE_SPATIAL

  if (type_arg == itMBR)

  {

    // handle spatial key

    return ...;

  }

#endif

  // handle scalar key

  return ...;

}

To turn GIS implementation into a data type plugin, let's do the following:

• move this code from Field_blob to Field_geom.
• move these dependencoes from item.h to item_geofunc.h:

  #include "gstream.h"

  #include "spatial.h"