WinRT: Do not repaint whole screen on every update

This provides an internal backing buffer on WinRTPageFlipper which is
mapped to the screen on each page flip.

Change-Id: Ie380d37ba93d02a0f78d5fefa1fc51a76ef760a6
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>