Bug 769976 - JPEG export ignores quality setting and subsampling fix.

Set use_orig_quality when both the quality and the subsampling are the
same as in the originally-imported jpeg.

Also improve subsampling initial selection: use the original subsampling
unless the default one is the best or the original one is the worst.
The current code was wrong and would often use the default subsampling
even when worse than the original one.