Fix gravity zones not being drawn in some cases

Namely, gravity zones would be rendered when the wrong wall (i.e. not Gravity wall) was selected if custom tools were present in the SC_WALL menu section. This was because their rendering was tied to a tool index into the SC_WALL menu section, rather than to a tool identifier.

src/gui/game/GameController.cpp

@@ -1111,8 +1111,11 @@ void GameController::SetActiveTool(int toolSelection, Tool * tool)
 	gameModel->SetLastTool(tool);
 	for(int i = 0; i < 3; i++)
 	{
-		if(gameModel->GetActiveTool(i) == gameModel->GetMenuList().at(SC_WALL)->GetToolList().at(WL_GRAV))
+		auto *activeTool = gameModel->GetActiveTool(i);
+		if (activeTool && activeTool->Identifier == "DEFAULT_WL_GRVTY")
+		{
 			gameModel->GetRenderer()->gravityZonesEnabled = true;
+		}
 	}
 	if (tool->Identifier == "DEFAULT_UI_PROPERTY")
 	{