gegl-tile: make tile COW lock-free

Copy-on-write for tile data is currently implemented using a shared
linked list among the cloned tiles, guarded by a global mutex.
Replace the list with a shared counter, which is manipulated
atomically, without a lock.

For tiles created using gegl_tile_new(), and for uncloned tiles,
the counter shares the same buffer as the tile data, so no
additional allocation is required.  Tiles created using
gegl_tile_new_bare() get a separately-allocated counter.