ThumbnailWrite method with storing the JpegSegments offset

[Gorbush]

This PR re-implements the method to write the Thumbnail into file without loading the bytes into memory during each image parsing.

I still need to extract thumbnails, and after removal of this method from ExifThumbnailDirectory - it is impossible, as offset is stored relative to the segment data, and reader with segment data is not available at that time.
This change is storing the offset of the segment in the root directory, so ti is possible to calculate the real thumbnail offset in the file ad hoc.

[Gorbush]

Hi,

Any comments on this?

[lfcnassif]

Any plans to integrate this PR? We can not upgrade the library because we need thumbnail extraction too, that method removal was a breaking change without pointing to alternative ways of getting the old functionality.