(svn r8333) -Codechange: when invalidating a build window list, set a flag instead of rebuilding the list and then rebuild it the next time it's redrawn

This should save CPU time in the (maybe unlikely) event that the list is invalidated more than once between two redraws

src/window.h

@@ -326,6 +326,7 @@ typedef struct {
 	byte sel_index;  // deprecated value, used for 'unified' ship and road
 	bool descending_sort_order;
 	byte sort_criteria;
+	bool regenerate_list;
 	EngineID sel_engine;
 	EngineID rename_engine;
 	EngineList eng_list;