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/screenshot.cpp

@@ -789,6 +789,7 @@ void SetupScreenshotViewport(ScreenshotType t, ViewPort *vp)
 			break;
 		}
 	}
+	UpdateViewportSizeZoom(vp);
 }
 
 /**