Redraw existing railroads when redrawing the complete layer (#1410)

## Description:

Fix bug that made railroads disappear when switching to dark mode.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/client/graphics/layers/RailroadLayer.ts

@@ -83,6 +83,10 @@ export class RailroadLayer implements Layer {
 
     this.canvas.width = this.game.width() * 2;
     this.canvas.height = this.game.height() * 2;
+
+    for (const [_, rail] of this.existingRailroads) {
+      this.paintRail(rail.tile);
+    }
   }
 
   renderLayer(context: CanvasRenderingContext2D) {