Export each tile as its own stream, and the grid information as a Stream Group
of type TILE_GRID.
This also enables exporting other stream items like thumbnails, which may be
present in non tiled HEIF images too. For those, the primary stream will be
tagged with the default disposition.

Based on a patch by Swaraj Hota
Signed-off-by: James Almer <jamrial@gmail.com>