Rounding problem when applying order discount before item discount

Description

Reported from:
https://world.episerver.com/forum/developer-forum/Episerver-Commerce/Thread-Container/2018/2/promotion-calculating-in-commerce-marketing/#188075

1. Go to the new Marketing UI.
2. Create an order discount of the type "Spend at least X amount and receive a discount off total order value.":
   Spend at least X = 5.00
   Get the following discount (Amount off) = 1.00
3. Create an Item discount with type "Buy at least X items from catalog entries and get related catalog entries at a discount.":
   Buy item X, get item X at (Amount off) = 1.00
4. Set the priority of the order discount higher than the item discount.
5. Add 3 items X (price = 11) to the cart and place the order.

Expected:
Item promotion 3, total discount 4, subtotal 30

Actual:
Item promotion 2.99, total discount 3.99, subtotal 30.01

• Note: The bug does not happen if the item discount has a higher priority than the order discount.