Add dzsave gainmap

[jcupitt]

TODO:

• add some tests

[jcupitt]

Test with:

$ dzsave ultra-hdr.jpg x --keep gainmap
$ vipsheader -f gainmap-data x_files/10/1_0.jpeg | base64 -d > map-tile.jpg

And verify that map-tile.jpg is the expected size and crop position. test_uhdr_dzsave does this test.

You need the --keep gainmap since dzsave strips all tile metadata by default, but of course you need to retain it here.