Give pull_var_clause() reject/recurse/return behavior for WindowFuncs too.

All along, this function should have treated WindowFuncs in a manner
similar to Aggrefs, ie with an option whether or not to recurse into them.
By not considering the case, it was always recursing, which is OK for most
callers (although I suspect that the case in prepare_sort_from_pathkeys
might represent a bug).  But now we need return-without-recursing behavior
as well.  There are also more than a few callers that should never see a
WindowFunc, and now we'll get some error checking on that.

src/backend/optimizer/plan/planner.c

@@ -3837,11 +3837,12 @@ make_group_input_target(PlannerInfo *root, List *tlist)
 	 * add them to the result tlist if not already present.  (A Var used
 	 * directly as a GROUP BY item will be present already.)  Note this
 	 * includes Vars used in resjunk items, so we are covering the needs of
-	 * ORDER BY and window specifications.  Vars used within Aggrefs will be
-	 * pulled out here, too.
+	 * ORDER BY and window specifications.  Vars used within Aggrefs and
+	 * WindowFuncs will be pulled out here, too.
 	 */
 	non_group_vars = pull_var_clause((Node *) non_group_cols,
 									 PVC_RECURSE_AGGREGATES |
+									 PVC_RECURSE_WINDOWFUNCS |
 									 PVC_INCLUDE_PLACEHOLDERS);
 	sub_tlist = add_to_flat_tlist(sub_tlist, non_group_vars);
 
@@ -4086,10 +4087,12 @@ make_window_input_target(PlannerInfo *root,
 	 *
 	 * Note: it's essential to use PVC_INCLUDE_AGGREGATES here, so that the
 	 * Aggrefs are placed in the Agg node's tlist and not left to be computed
-	 * at higher levels.
+	 * at higher levels.  On the other hand, we should recurse into
+	 * WindowFuncs to make sure their input expressions are available.
 	 */
 	flattenable_vars = pull_var_clause((Node *) flattenable_cols,
 									   PVC_INCLUDE_AGGREGATES |
+									   PVC_RECURSE_WINDOWFUNCS |
 									   PVC_INCLUDE_PLACEHOLDERS);
 	new_tlist = add_to_flat_tlist(new_tlist, flattenable_vars);