Move into a standalone meta-wayland-surface-role-cursor.[ch], and
make generic enough to work for pointe and additional (eg. tablet)
cursors.

Most notably, the sprite is now kept completely internal to the
cursor role, and updates are routed through the given
MetaCursorRenderer (which may be the default one for the pointer,
or something else).

The way cursor updates after cursor surface destruction has also
been reworked, the pointer will just keep track of the last cursor
surface, so older surfaces being destroyed don't trigger pointer
rechecks/updates.