Subdataset overview filenames stored with absolute path making them non-portable

[gaopeng]

The issue is that the OVERVIEW_FILE value saved out is a local absolute path, which can be broken if the base image is accessed via UNC path.

For example, a local path like:

E:\Temp\Kabul_uncompressed\other\08MAY18184148-P1BS-005765570010_02_P002.NTF_0.ovr

is saved as OVERVIEW_FILE, and when the image is accessed using a UNC path on a network,

rberger\temp\Kabul_uncompressed\other\08MAY18184148-P1BS-005765570010_02_P002.NTF

the ovr path E:\Temp\... is not valid.

It also makes it unnecessarily difficult to move filesets around.