Code review comment for lp://staging/~nick-dedekind/qtmir/multiwindow.textures

> Since CompositorTexture is only meant to be created and filled in by
> CompositorTextureProvider, it makes sense to make its constructor and
> setTexture methods private and declaring CompositorTextureProvider its friend.
> That would make roles more clear when you read the header

Aside from this nit, code looks good and unity8 still works.