In GeglTileHandler, add new gegl_tile_handler_get[_source]_tile()
functions, which fetch a tile with a given coordinates from the
tile handler (or its source).  If the tile doesn't exist, these
functions return a newly-created tile.

The functions take an additional preserve_data argument.  When it
is FALSE, the existing tile data doesn't have to be preserved,
which allow us to perform several optimizations:  Instead of trying
to fetch the tile using a TILE_GET command, we look up the tile in
the cache.  If the tile is not in the cache, we return a new tile
-- this avoids unnecesarily reading the tile from the storage, or
generating it.  If the tile is in the cache, we fully damage it, so
that if it's uncloned, its existing data won't be copied (see last
commit).