[FIX] point_of_sale: order total round globally
[FIX] point_of_sale: order total round globally

Before this commit, the amount total of an order in round_globally
was simply the sum of all taxes amounts, then rounded
This is plain wrong, as the accepted semantics of round globally is that:
- each tax for each line is computed without rounding
- then each taxes is summed across all order lines
- then that amount is rounded
- then all those rounded tax amount are summed together

After this commit, this semantics is implemented

closes odoo/odoo#35211

Signed-off-by: Lucas Perais (lpe) <42d29efea96f3992aa24782120ec2948e42548fa@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo