QTiffHandler: port Private to NSDMI

This is simpler, and lets readers confirm with one look that all
fields are properly initialized (QSize has a default ctor itself). It
also fixes a 0 used as nullptr.

Amends various commits, so just picking back to avoid conflicts in
this security-critical component, so any security fix can later travel
back faster.

Pick-to: 6.12 6.11 6.8
Change-Id: I27dbc218612d63c34609573f86b230da29ad3ad5
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>