[sane.ds try2 1/3] Get resolution from sane, instead of hard coding -1.

[Juan Lang]

Hi again, Jeremy,

> With this change, sane.ds will not compile unless SONAME_LIBSANE is
> defined.

This remark is incorrect, sorry.  I only read the patch, I didn't read
ds_image.c first.  This change is correct.

> The other comment is, is adding a new file (option.c) really
> necessary?  If you're planning to expand it a lot, perhaps, but just
> for this one small function it looks like overkill to me.

This remark still stands.
--Juan