Set the z-spacing in a TIFF to match x-spacing

There is no way to specify the z-spacing, and it seems more logical
to default to something matching the x-spacing rather than defaulting
to 1.0. This will of course result in 1.0 when nothing is specified
in the file.

Change-Id: I6e1f9e4f2f294844a6ed84ba1aa439399f5eb957
Merge topic 'tiff-z-spacing' into master

cebeddb1 Set the z-spacing in a TIFF to match x-spacing