Be conservative with atlas allocation for Cover windows.

These are usually small and require a very small amount of textures.

Change-Id: I51a1d2965c37ac39c1f1f70d9bb6d7baf41e0562
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>