Fix #7847: Use ViewportSign coordinates for sign Kdtree coordinates (#7849)

Ensure the same coordinates are used for station/town/player signs regardless of how the landscape changes below it after the coordinates were first determined.

By keeping track of whether each ViewportSign is valid for Kdtree use (and only ever registering the viewport sign when the object is valid) a lot of code can be simplified and become more robust at the same time.

src/station.cpp

@@ -162,7 +162,7 @@ Station::~Station()
 	CargoPacket::InvalidateAllFrom(this->index);
 
 	_station_kdtree.Remove(this->index);
-	_viewport_sign_kdtree.Remove(ViewportSignKdtreeItem::MakeStation(this->index));
+	if (this->sign.kdtree_valid) _viewport_sign_kdtree.Remove(ViewportSignKdtreeItem::MakeStation(this->index));
 }