Merge pull request #21 from giucam/mer-5.4

MER: Make sure to only prevent cover surfaces from hiding
Make sure to have valid textures

The texture of QWaylandSurfaceItem's texture provider was updated in
QWaylandSurfaceItem::updatePaintNode(). That is fine as long as the
texture is accessed after the updatePaintNode() call, but if not we
would access an invalid texture.
That could happen when another QQuickItem subclass was accessing the
textire in its updatePaintNode(). We don't have any guarantee on the
order of the updatePaintNode() calls, so the other item could be
updated before the QWaylandSurfaceItem.