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/smallmap_gui.cpp

@@ -1735,6 +1735,15 @@ public:
 	{
 		for (NWidgetBase *child_wid = this->head; child_wid != nullptr; child_wid = child_wid->next) child_wid->Draw(w);
 	}
+
+	void FillDirtyWidgets(std::vector<NWidgetBase *> &dirty_widgets) override
+	{
+		if (this->base_flags & WBF_DIRTY) {
+			dirty_widgets.push_back(this);
+		} else {
+			for (NWidgetBase *child_wid = this->head; child_wid != nullptr; child_wid = child_wid->next) child_wid->FillDirtyWidgets(dirty_widgets);
+		}
+	}
 };
 
 /** Widget parts of the smallmap display. */