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.

core/math/rect2.h

@@ -393,11 +393,12 @@ struct Rect2i {
 	operator String() const { return String(position) + ", " + String(size); }
 
 	operator Rect2() const { return Rect2(position, size); }
+
+	Rect2i() {}
 	Rect2i(const Rect2 &p_r2) :
 			position(p_r2.position),
 			size(p_r2.size) {
 	}
-	Rect2i() {}
 	Rect2i(int p_x, int p_y, int p_width, int p_height) :
 			position(Point2(p_x, p_y)),
 			size(Size2(p_width, p_height)) {