cursor-renderer: We always use a MetaStage
cursor-renderer-native: Don't move the cursor when we don't need to
cursor: Correct switching between fallback and backend cursors

When switching from the stage cursor to the native cursor, we
forgot to repaint the stage to get rid of the old cursor. Fix
this by having the abstract cursor renderer class track whether
we're using the backend, rather than doing chain-up shenanigans.
cursor: Add a META_CURSOR_NONE

This is for cases where we have a MetaCursorReference that does
not have an X cursor equivalent... we have to return something!
x11: Add simple cursor renderer