refactoring of mobile demo so that it no longer uses an embedded startup texture

Instead, it's now displaying the progress bar with the classic display list; to avoid flickering, the startup-image is loaded using a Loader with 'FileStream.readBytes'. Thanks to Josh for making me aware of this solution!
refactoring of scaffold so that it no longer uses an embedded startup texture
moved the 'updateNativeOverlay' code into the 'onEnterFrame' callback

That way, the overlay has the correct size even before you call "start" on Starling. That's useful when using the conventional display list e.g. for the loading screen.