This code could be genericized, but as the other 6" sinfonia models
automatically combine prints it's not terribly useful.
The 6" printers don't appear to support a 5x7-div2 cut size, so
this remains only useful for 4x6. Similarly, we can't do a 6x8-div4.
But as all 6" models can rewind, this is only a print speed optimization
rather than a media use optimization!
* Decision logic for combining jobs is global, with a per-backend
combine_job() call for the nitty-gritty
* Job combining code now handles multipage jobs as well as
multiple copies of a given page
* Will attempt to batch multipage jobs
* Automatically invoked every time a job is appended to a joblist
* Only used by DNP and Mitsu D70 backends (so far)