Since this signal is in a hot path during input handling, it makes sense
not to have this be a signal at all, currently most of the time spent in
it is in GLib signal machinery itself.
Replace it with a function/user data pair that are set on the sprite
itself. Only the places that create an sprite are interested in hooking
one ::prepare-at behavior per sprite, so we can do with a single pair.
This makes meta_cursor_sprite_prepare_at() inexpensive enough.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1915>
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