Fix rendering on Retina displays

Since the QWindow returned by the RWHV delegates was always 0 we never
actually propagated a valid WebScreenInfo to chromium.
Additionally the painting and scrolling in the backing store had to be
fixed so that the device pixel ratio is taken into account.

Change-Id: I22dc135e8e090362201292863ed911464b9fc133
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>