Move tower background update code to logic functions

Otherwise it'll go really really quickly, which is not good.

desktop_version/src/editor.cpp

@@ -3622,6 +3622,10 @@ void editorlogic()
             break;
         }
     }
+    else if (!game.colourblindmode)
+    {
+        graphics.updatetowerbackground();
+    }
 
     if (graphics.fademode == 1)
     {