bugfix: gridlines always span entire window

To fix this we ensure Viewport (w,h) always matches screen/window
dimensions.

We choose to track viewport (x,y) by file but (w,h) globally. Seems a
little less jarring than (w,h) by file.

0012-on.initialize

@@ -6,6 +6,7 @@ on.initialize = function(arg)
 	if Settings[Filename] then
 		Viewport = Settings[Filename].viewport
 	end
+	Viewport.w, Viewport.h = App.screen.size()
 	load_graph_from_disk()
 	A()
 end