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.