- 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.