qt: do not let Qt to create implicit layer for artist top banner and blur only once

Akin to !6573, but in this case we don't need TextureProviderItem (!6560), because the source image is invisible when the blur effect is shown. It can simply use Stretch without side effects as the source size height is calculated to consider the aspect ratio independent of fill mode because only the width is set, so the texture is generated as the blur effect desires unless tiling is needed.

We can not do this in !6573, because there we re-use the texture used by a visible image for the blurred background. If a visible image has fill mode stretch, it will be stretched, and we don't want that in the player view.

I have checked with GammaRay, and I can confirm that this way an implicit layer is not created. It also has the benefit of not having to blur the source each time the size changes. I simply use a clip node and adjust the blur viewport manually.

Unfortunately, we can not do this with older Qt versions, because if the source image changes fill mode from stretch or to stretch where it is fed to a blur effect, the effect either causes crash (layering needed to not needed, so Tile -> Stretch) or does not work (layering not needed to needed, so Stretch -> Tile).

Request review @chub.