GEGL_TILE_COPY requests a tile source to copy a tile to a given
destination buffer (or to the same buffer to which the tile source
belongs), at given destination tile coordinates.  The tile source
and the destination buffer are assumed to be tile-compatible (i.e.,
having the same tile dimensions and format).  The tile source
should return a boolean value, indicating whether the tile has been
copied.

This is mostly meant to be implemented by the cache handler, and by
tile backends (the swap backend, in particular).  It allows
backends to implement a cheap copy operation, avoiding the need to
actually fetch the tile.  For example, at the moment, copying a
swapped-out tile between two buffers, both of which are using the
swap backend, requires reading the tile from disk into memory
first, only to COW the result.  This might not be an issue if the
same tile is known to be accessed shortly after, but when doing a
tentative copy, usually of an entire buffer (as is done in GIMP in
several cases), this incurs a very noticeable overhead.