WIP: rts8891: Umax Astra 4400/4450 support
I'm resuming work on Umax Astra 4400/4450 support from 2015.
The current state is:
- quick preview works in xsane (emulated 100dpi)
- scanning works at 150dpi and 300dpi (not at 600/1200dpi)
- calibration does not work
- no support for UTA