Change source item pulling logic: previously 2 leaves items
were taken at once from the same stack, so only identical types
were used to produce Biogas (remaining single items could fill
up source tray). Now items are taken one by one, allowing 2
different plant leaves to form 1 Biogas unit. That cleans up
inventory as well - maximum one item is left there when production
ends.
Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>
