Issue #8339: Fix default TIFF export settings
This attempts to resolve issue #8339 (closed), created by the !634 (merged) merge request.
Currently in 2.10, GIMP only sets the BIGTIFF option to FALSE if the libtiff version does not support BIGTIFF.
Since regular TIFF is the more common export format, this patch makes GIMP initialize the BIGTIFF option to FALSE regardless. It then checks the prior export settings to optionally set the BIGTIFF export option back to TRUE (again, assuming BIGTIFF is supported by the user's libtiff).