Add an internal _gimp_tile_ref_noinit() function, which increases
the ref-count of a tile *without* initializing its data (in
particular, without reading its data from GIMP, or zeroing it.)
Use this function, instead of gimp_tile_ref(), when storing a tile
in GimpTileBackendPlugin, to avoid unnecessarily reading the tile
data from GIMP.

(cherry picked from commit 5ffdb9aa)