Fix excavator delivering not full buckets

When an excavator mines the final scoop of a tile, the bucket doesn't get full (20 units for excavators as an example). So instead of filling the bucket in another mining spot, the excavator delivers the half bucket to a truck, thus unaligning the exact ratio of units its bucket has to trucks cargo. It will forever take one more bucket to entirely fill any truck, unless an unlikely scoop aligns it again. To fix this, the excavator should look for another mining location to fill its bucket, and only deliver the half content to trucks if it has no other location to work.