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.

Draft because of the last sentence, maybe it's worth doing in this MR? wayland stuff is easily doable, but GLX and EGL/X11 need more work, I don't think it's worth it right now