[FIX] sale: subtotal computation and display

Steps to reproduce:
- Install Accounting and Sale
- Enable the developer mode.
- Go to Settings > Users & Comp > Your user, edit to check both "Tax
Display B2B" and "Tax Display B2C" boxes.
- Go to Settings > General Settings > Accounting > Reporting, check the
"Add totals below sections" box.
- Create a sale order in the sale module, the sale order must have at
least one section with several products.
- Print the sale order. The subtotal amounts are wrong.

There are two different subtotals amount to compute, one for the
*Amount* column and one for the *Total Price*. The actual computation
did not store the two amount in different variables. Multiple template
are broken in the same way.

Fine tunning of 716ff76d

Co-authored-by: Nans Lefevre <len@odoo.com>

opw-1950833

closes odoo/odoo#32148

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>