xwayland: Move sprite invalidation logic into mipointer

This creates a function that invalidates the current sprite and forces
a sprite image reload the next time the sprite is checked, moving that
logic out of the xwayland sources and allowing the miPointerRec
structure to be removed from the server API.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>