3.0 - Discounts are not being stored correctly in the adjustments table

[robincornett]

Bug Report

Expected behavior

When I purchase a variably priced product with a percentage based discount, I expect the discount subtotal/total amounts to be correct in the adjustments database table.

Actual behavior

The discount is not calculated correctly and is either 0, or if a non-variable product is in the order, the discount is the amount for just the one item.

Steps to reproduce the behavior

1. Create a product with variable pricing.
2. Create a percentage based discount.
3. Add the variable product to your order and apply the discount.
4. Complete the order.
5. Note that the receipt appears to show the correct information and totals. However, if you check the edd_order_adjustments table for that order ID, the discount amount is incorrect. Additionally, when you go to edit the order in the admin, the discount does not show correctly, as it's calculated from the database, not the payment object (the receipt uses old payment functions 3.0 - update receipt shortcode template to use order object #8508).

Information (if a specific version is affected):

PHP Version: 7.4
EDD Version (or branch): release/3.0
WordPress Version: 5.7

Any other relevant information:
I'm not sure if the issue is in the edd_get_item_discount_amount function or in how the cart_details are built, but I think the failure point is here:

https://github.com/easydigitaldownloads/easy-digital-downloads/blob/af1e1439ff1eb60443bcd5cd8460f091bcaaaa0c/includes/discount-functions.php#L1028-L1030

It looks like a similar issue was fixed in the cart for 2.10 in issue #8246, in which case we need to apply the same fix to the edd_get_item_discount_amount function.

This also affects an open PR in Invoices to show discounts.

Note: marking for beta2 as it's also affecting #8075.