[FIX] sale: round untaxed amount to invoice after computations

Suppose such a SO line:
- Unit Price: 541.26
- Qty: 38
- Discount: 2%

The line amount is computed thanks to application of the discount on the
price, then the multiplication of the result with the quantity. The
rounding eventually happens after these steps:
https://github.com/odoo/odoo/blob/254b2a0840d3e817cc6062c483b5213c10088af5/addons/sale/models/sale.py#L1013-L1024
https://github.com/odoo/odoo/blob/b76e9ef658bde0178fa1660b6ad27b880e91632a/addons/account/models/account.py#L1138


Therefore, in the above case, we have:
```
Total = round(541.26 * 0.98 * 38)
      = round(530.4348 * 38)
      = round(20156.5224)
      = 20156.5224
```

However, when confirming the SO and setting the delivered quantity, the
Untaxed Amount To Invoice is computed using the unit price with both the
discount and the rounding already applied:
```
Total = round(541.26 * 0.98) * 38
      = round(530.4348) * 38
      = 530.43 * 38
      = 20156.34
```
As a result, the amount is not the same than the first one, this is
incorrect.

OPW-2525975

closes odoo/odoo#74625

X-original-commit: 1622ee96
Signed-off-by: Adrien Widart <adwid@users.noreply.github.com>