Turn WindowThumbnail into a texture provider
This gives a significant speed boost if windowThumbnails are then used via a ShaderEffect without having to render to an intermediate framebuffer.
It also gives the ShaderEffect the texture at the resolution of the source, rather than the size of the Item.
Memory management of the node created in WindowThumbnail::textureProvider is a bit quirky, but a pattern copied in multiple places in Qt (QQuickFrameBufferObject for example) so should be safe.