qt: get rid of the layer used for tiling in artists page

Do tiling at sampler level than layer level, thanks to the texture view. This allows us to save video memory.

The first commit fixes the problem of layer remaining once initialized (switch or late initialization case), in case we revert from the new approach where layer is not used at all.

I also fix being unable to reset sub-rect in texture view/provider.

Before:

After:

I manually tried to pick the tiling parameters to make it look similar to before, I can change it if wanted:

                textureSubRect: blurEffect.sourceNeedsTiling ? Qt.rect(blurEffect.width / 8,
                                                                       blurEffect.height / 8,
                                                                       blurEffect.width * 1.25,
                                                                       blurEffect.height * 1.25) : undefined