Window functions: make "ORDER BY window_func" work

- When window functions are present, JOIN::simple_order should be set
  to FALSE. (Otherwise, the optimizer may attempt to do a "pre-sorting"
  on the first join_tab. Which can work in some cases, but generally
  isn't)

- filesort tries to only read table fields that it requires. Window
  function requires its temp.table field.  In order to pass this info
  to filesort, added an implementation of Item_window_func::
  register_field_in_read_map.

sql/sql_select.cc

@@ -1846,6 +1846,7 @@ JOIN::optimize_inner()
   //TODO this could probably go in test_if_need_tmp_table.
   if (this->select_lex->window_specs.elements > 0) {
     need_tmp= TRUE;
+    simple_order= FALSE;
   }
 
   /*