Smoother link of processing cycles to ABM cycles

Smoother link of processing cycles to ABM cycles

Make the generic processing machine code willing to complete more than
one processing cycle in one ABM cycle, and more generally to carry
over leftover processing effort after completing a processing cycle.
The src_time meta item now represents accumulated processing effort
(time multiplied by speed) in a scaled form (to retain fractions),
rather than just time in integral seconds.  This affects the MV furnace,
with speed 4 and most recipe times being 3 s, and will be essential for
faster furnaces.