core: Move surface texture factory functions to RenderBackend

The goal is to create surface items for things that are not in the
workspace scene. RenderBackend perhaps is not a great place for these
factory functions. On the other hand, this change merely rewires code
from Scene to RenderBackend. I think that in distant future we could
make surface items pick surface texture type on their own, for what it's
worth that's what we would do in QtQuick.