The DnD surface will be always updated/redrawn when the cursor position
changes, and it will never go through backend-specific handling of hw
cursors, so the stage will always need updating when a drag is in effect.
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.
