There was no implementation of the virtual method print()
for the Item_window_func class. As a result for a view
containing window function an invalid view definition could
be written in the frm file. When a query that refers to
this view was executed a syntax error was reported.
Make window functions work with an empty over clause by forcing
a sort on the first column of the current join_tab. This is a temporary
fix until we get window functions to work with big tables.
Perform only one table scan for each window function present. We do this
by keeping keeping cursors for each window function frame bound and
running them for each function for every row.
