qml: expose the texture provider in `ImageExt`

I realized that this was missing in !7485 (diffs, comment 491835), where the effect source could be the texture provider so that the item is not layered unnecessarily.

With DualKawaseBlur this would be obvious (the effect would not work unless layer.enabled: true is used or ShaderEffectSource is used explicitly), since it strictly requires a texture provider as the source item (purposefully), but MultiEffect or FastBlur creates an implicit layer if the source item is not a texture provider. In fact, they even create a layer when the source is texture provider as they deem necessary (which is a very bad idea in my opinion, because the effect can not know whether the source wants layering or not), such as when the source item has children.