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/network/network_gui.cpp

@@ -206,6 +206,19 @@ public:
 		return nullptr;
 	}
 
+	void FillDirtyWidgets(std::vector<NWidgetBase *> &dirty_widgets) override
+	{
+		if (this->base_flags & WBF_DIRTY) {
+			dirty_widgets.push_back(this);
+		} else {
+			int i = 0;
+			for (NWidgetBase *child_wid = this->head; child_wid != nullptr; child_wid = child_wid->next) {
+				if (!this->visible[i++]) continue;
+				child_wid->FillDirtyWidgets(dirty_widgets);
+			}
+		}
+	}
+
 	/**
 	 * Checks whether the given widget is actually visible.
 	 * @param widget the widget to check for visibility