Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

modules/gdnavigation/nav_map.cpp

@@ -33,6 +33,7 @@
 #include "core/os/threaded_array_processor.h"
 #include "nav_region.h"
 #include "rvo_agent.h"
+
 #include <algorithm>
 
 /**
@@ -41,16 +42,6 @@
 
 #define USE_ENTRY_POINT
 
-NavMap::NavMap() :
-		up(0, 1, 0),
-		cell_size(0.3),
-		edge_connection_margin(5.0),
-		regenerate_polygons(true),
-		regenerate_links(true),
-		agents_dirty(false),
-		deltatime(0.0),
-		map_update_id(0) {}
-
 void NavMap::set_up(Vector3 p_up) {
 	up = p_up;
 	regenerate_polygons = true;