TODO UMAX BACKEND FOR SANE
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 * add backend internal callibration support for Astra 2200

 * change set_window if (dev->do_color_ordering != 0) {..}
   one of the settings makes an error for Astra 6X0S scanners

 * include push-button function to SANE_START (?)

 * add builtin halftone pattern selection

 * add halftone pattern doenload, dimension selection


 - I will NOT add support for parallel-port and USB scanners to this backend!