[18.0][FIX] sale_global_discount: discount amounts with round_globally rounding method

[LorenzoC0]

This patch disables the calculation of global discount amounts if none is selected and computes discount amounts consistently from per-line subtotals instead of mixing them with globally rounded order totals.

You can trigger a -0,01€ discount without any global discount selected (or a global discount set to 0%) with these steps:

1. Create a sales order.
2. Set the unit price precision to 3 decimal places.
3. In the sales order, create 4 lines:
   • Product 1 - qty 3 - unit price 3.460;
   • Product 2 - qty 1 - unit price 25,000;
   • Product 3 - qty 6 - unit price 4,713;
   • Product 4 - qty 6 - unit price 4,501.
4. Do not set a global discount.
5. Check the amount of the global discount applied.

[eLBati]

Thanks