image/jpeg: fix extended sequential Huffman table selector (Th).

image/jpeg: fix extended sequential Huffman table selector (Th).

Previously, the package did not distinguish between baseline and
extended sequential images. Both are non-progressive images, but the Th
range differs between the two, as per Annex B of
https://www.w3.org/Graphics/JPEG/itu-t81.pdf

Extended sequential images are often emitted by the Guetzli encoder.

Fixes #19913

Change-Id: I3d0f9e16d5d374ee1c65e3a8fb87519de61cff94
Reviewed-on: https://go-review.googlesource.com/41831
Reviewed-by: David Symonds <dsymonds@golang.org>