Only defer duplicates in the first zoom. Limit deferral depth.

Make tmpdir a global to keep from needing to pass it around

main.hpp

@@ -54,6 +54,7 @@ extern int retain_points_multiplier;
 extern size_t maximum_string_attribute_length;
 extern std::string accumulate_numeric;
 extern unsigned long long preserve_multiplier_density_threshold;
+extern const char *tmpdir;
 
 struct order_field {
 	std::string name;