In GimpTileBackendPlugin, change the default tile multiplier,
specifying the ratio between the backend tile-size, and GIMP's
tile-size, from 2 to 1.  Since we're reading/writing each GIMP tile
using a separate command anyway, using a large multiplier doesn't
provide any benefits, while it does have drawbacks.  In particular,
it reduces the chance that a write operation will affect an entire
tile, which allows us to avoid reading the tile data from GIMP.

(cherry picked from commit a5e2945b)