[FIX] point_of_sale: tip value not a discount
[FIX] point_of_sale: tip value not a discount

When we are adding a tip, the unit_price of the tip product is changed
on the line, but not the lst_price, that lead to show the difference
between the lst_price and unit_price as a discount on the receipt if the
pricelist is configured to show its price difference as a discount.

So we are now also changing the lst_price.

TASK-ID: 2058650
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo