screenmanager: fix shader-based transition when they are not used in fullscreen. Now it works perfectly even when used in a specific pos/size.
screenmanager: add a new ScreenManager.switch_to method to add the screen into children, set transition options, do the switch, and remove the previous screen. It also handle name conflict. closes #573, #1045
Merge branch 'master' of ssh://github.com/kivy/kivy