Forage Wagon Hay Lock

PURPOSE
Prevents hay already collected in a forage wagon from being overwritten by grass or by another hay type.

BEHAVIOUR
- More of the SAME hay type collects normally.
- Normal grass is ignored while hay is loaded.
- Fresh Clover and fresh Alfalfa are ignored while hay is loaded when those fill types exist.
- A DIFFERENT hay type is ignored while hay is loaded.
- Empty wagons and wagons containing non-hay material retain normal game behaviour.

SUPPORTED HAY TYPES
- Hay: DRYGRASS_WINDROW
- Clover Hay: DRYCLOVER_WINDROW
- Alfalfa Hay: DRYALFALFA_WINDROW