[FIX] point_of_sale: compute the tax based on the rounded price
[FIX] point_of_sale: compute the tax based on the rounded price

Before this, we would compute taxes based on quantity * price. In
account.tax (used by the POS backend) however taxes are computed on a
rounded (based on currency precision) quantity * price. This would cause
issues as orders in the backend would have a different total price as
calculated in the frontend.

An example that used to trigger the problem:

Order with orderline with product with price of 5, excluded tax of 21%
and a quantity of 0.985. This would cost 5.96 in the frontend and 5.97
in the backend.

stephen144/odoo,laslabs/odoo,bplancher/odoo,optima-ict/odoo,ygol/odoo,dfang/odoo,Elico-Corp/odoo_OCB,ygol/odoo,storm-computers/odoo,sysadminmatmoz/OCB,ygol/odoo,storm-computers/odoo,storm-computers/odoo,bplancher/odoo,bplancher/odoo,sysadminmatmoz/OCB,optima-ict/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,hip-odoo/odoo,microcom/odoo,dfang/odoo,stephen144/odoo,stephen144/odoo,hip-odoo/odoo,stephen144/odoo,hip-odoo/odoo,optima-ict/odoo,stephen144/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,optima-ict/odoo,ygol/odoo,stephen144/odoo,laslabs/odoo,storm-computers/odoo,microcom/odoo,dfang/odoo,microcom/odoo,dfang/odoo,sysadminmatmoz/OCB,microcom/odoo,optima-ict/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,bplancher/odoo,bplancher/odoo,sysadminmatmoz/OCB,dfang/odoo,ygol/odoo,ygol/odoo,sysadminmatmoz/OCB,storm-computers/odoo,hip-odoo/odoo,storm-computers/odoo,microcom/odoo,hip-odoo/odoo,laslabs/odoo,ygol/odoo,dfang/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,optima-ict/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,microcom/odoo