Codechange: Use functor for Kdtree's XYFunc. (#13074)

Kdtree uses a function pointer and incorrectly calls it a functor. The function pointer needs to be passed on instantiaton.

Instead, use an actual functor. This simplifies instantiation.

src/viewport.cpp

@@ -102,7 +102,7 @@
 Point _tile_fract_coords;
 
 
-ViewportSignKdtree _viewport_sign_kdtree(&Kdtree_ViewportSignXYFunc);
+ViewportSignKdtree _viewport_sign_kdtree{};
 static int _viewport_sign_maxwidth = 0;