Completing fix from commit 3cb70e6.
Checking further into libtiff logs, I can see another version of the
warning message for private tags which was used before libtiff 3.7.
For sake of completeness, let's check against this version of the
warning as well.
This is a regression from bug 131975. Any unknown tag over 32768 is not
an error. This is a reserved zone where it is allowed to create custom
tags.
The warning indeed changed since libtiff 4.0.0alpha where it has become:
"Unknown field with tag %d (0x%x) encountered"
This explains why it was not recognized anymore.
Using code from gegl:tiff-load and gegl:tiff-save which has a mode for
local buffering if the input/output streams don't support seeking.
Unfortunately this code is broken and is disabled for now.
