(svn r16694) -Fix [FS#2995] (rgradual loading, rnewindustries): only pay for whatever has been actually unloaded and perform the payment when unloading has finished. This fixes, amongst others:
* cheating by starting to unload and after getting paid rushing to the depot to get sold (or unloading, loading and getting paid again for the remainder)
* cargo being dropped onto a station at the moment a stockpiling industry doesn't accept it anymore
* industries getting cargo that has not been unloaded yet and subsequently dumping it back on the station in one go
Note: you will now get paid after the unloading has finished, so you'll have to wait a bit longer for 'your' money.