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.h

@@ -23,7 +23,7 @@ enum StationCoverageType {
 };
 
 int DrawStationCoverageAreaText(int left, int right, int top, StationCoverageType sct, int rad, bool supplies);
-void CheckRedrawStationCoverage(const Window *w);
+void CheckRedrawStationCoverage(Window *w);
 
 void ShowSelectStationIfNeeded(const CommandContainer &cmd, TileArea ta);
 void ShowSelectWaypointIfNeeded(const CommandContainer &cmd, TileArea ta);