UpdateScreen sets the layout of the applet and is responsible of update the screen when the
mapStructur is updated or when the client moves.
The client itself is always at the center of the screen. When UpdateScreen wants to show the map
on the screen, it doesn't draw all the 9 blocks but checks which blocks should appear on the
screen and draw them.
UpdateScreen extends Canvas and uses double buffers to draw the image on the screen. It first sets the layout of the screen and the method paintApplet is responsible of drawing the screen.