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/main_gui.cpp

@@ -149,6 +149,7 @@ bool DoZoomInOutWindow(ZoomStateChange how, Window *w)
 	if (vp != nullptr) { // the vp can be null when how == ZOOM_NONE
 		vp->virtual_left = w->viewport->scrollpos_x;
 		vp->virtual_top = w->viewport->scrollpos_y;
+		UpdateViewportSizeZoom(vp);
 	}
 	/* Update the windows that have zoom-buttons to perhaps disable their buttons */
 	w->InvalidateData();
@@ -183,6 +184,7 @@ void FixTitleGameZoom()
 	vp->zoom = _gui_zoom;
 	vp->virtual_width = ScaleByZoom(vp->width, vp->zoom);
 	vp->virtual_height = ScaleByZoom(vp->height, vp->zoom);
+	UpdateViewportSizeZoom(vp);
 }
 
 static const struct NWidgetPart _nested_main_window_widgets[] = {
@@ -613,5 +615,4 @@ void GameSizeChanged()
 	_cur_resolution.height = _screen.height;
 	ScreenSizeChanged();
 	RelocateAllWindows(_screen.width, _screen.height);
-	MarkWholeScreenDirty();
 }