In GeglTileBackendSwap, when compressing a queued tile while
waiting for sufficient queue space, we allocate a tile-sized buffer
to hold the compressed data (since we don't know the compressed
data size in advance), hence the cost of the compressed tile in the
queue is equal to the full tile size.  However, we would previously
take its cost to be the compressed data size, which can be much
lower, causing us to over-fill the queue.  Use the full tile size
as the cost of queued compressed tiles to fix that.

Furthermore, don't compress cloned queued tiles, whose cost is less
than their data size, since this would increase their cost (these
tiles will still be compressed upon writing.)