[FIX] pos_discount: change customer

- Enable general discounts for POS
- Create a POS session
- Add a product
- Create a general discount of 10%
- Change the customer

The discount is reset to the product price.

When changing the customer, the prices are reset:

https://github.com/odoo/odoo/blob/3b1699e02d4483913159681bf7e10839e6493244/addons/point_of_sale/static/src/js/screens.js#L1251

In the case of a global discount, the price is therefore reset to the
product price in:

https://github.com/odoo/odoo/blob/3b1699e02d4483913159681bf7e10839e6493244/addons/point_of_sale/static/src/js/models.js#L2304

To avoid this, we set the price as manually set. Conceptually, this is
indeed the case since we manually chose the percentage, therefore the
price.

opw-2009761

closes odoo/odoo#34416

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/pos_discount/static/src/js/discount.js

@@ -42,10 +42,16 @@ var DiscountButton = screens.ActionButtonWidget.extend({
         }
 
         // Add discount
+        // We add the price as manually set to avoid recomputation when changing customer.
         var discount = - pc / 100.0 * order.get_total_with_tax();
 
         if( discount < 0 ){
-            order.add_product(product, { price: discount });
+            order.add_product(product, {
+                price: discount,
+                extras: {
+                    price_manually_set: true,
+                },
+            });
         }
     },
 });