Change how dirty screen, window and viewport areas are tracked for later redrawing

Track dirty viewport areas seperately form general screen redraws.
Maintain a dirty block grid per viewport, with a smaller block size.
Use even smaller block size in viewport map mode.

Use a rectangle array for general screen redraws instead of a block grid.

Add a dirty bit to windows and widgets, to simplify the common case
of repainting a whole window or widget, without catching
neighbouring windows or viewports.

src/station_gui.cpp

@@ -126,7 +126,7 @@ static void FindStationsAroundSelection()
  * If it is needed actually make the window for redrawing.
  * @param w the window to check.
  */
-void CheckRedrawStationCoverage(const Window *w)
+void CheckRedrawStationCoverage(Window *w)
 {
 	/* Test if ctrl state changed */
 	static bool _last_ctrl_pressed;