qt: do not let Qt to create implicit layer for player blurred background

Based on !6560, only the last commit is relevant.

!6517 (merged) already got rid of the explicit layer that we used, but now we also need a way to tell Qt that we don't need an implicit layer. Unfortunately, Qt does not offer a way, but simply decides itself (https://github.com/qt/qtdeclarative/blob/6.8.0/src/effects/qgfxsourceproxy.cpp#L143). Fortunately, there is the new TextureProviderItem that we can use here as an indirection with no incurred texture copy.

I checked with GammaRay that indeed using TextureProviderItem makes Qt to not create an implicit layer, and I have not found any side effects (this is expected, layering should not be necessary here).