[FIX] point_of_sale: tax calculation

Inconsistencies can arise between the tax calculation performed in the
PoS and in the backend (e.g. in a SO). For example:
- Unit price: 9.479
- Tax: excluded, 5.5 %
- Buy 5 units
- Round globally

In the backend, the result is 47.40 + 2.60 = 50.00
In the PoS, the result is 47.40 + 2.61 = 50.01

We make the PoS apply the same logic than in the backend. First, at the
level of the `compute_all` method, especially to use the appropriate
rounding precision before starting the calculation (so we move the
rounding of `total_excluded` after the rounding precision adjustment).
Then, by calculating the taxes amount as the difference between
`priceWithTax` with `priceWithoutTax`. This is mandatory since they
contain the result of `compute_all` in which the rounding logic has
already been applied.

opw-680891

addons/point_of_sale/static/src/js/models.js

@@ -1337,13 +1337,14 @@ exports.Orderline = Backbone.Model.extend({
     },
     compute_all: function(taxes, price_unit, quantity, currency_rounding) {
         var self = this;
-        var total_excluded = round_pr(price_unit * quantity, currency_rounding);
-        var total_included = total_excluded;
-        var base = total_excluded;
         var list_taxes = [];
+        var currency_rounding_bak = currency_rounding;
         if (this.pos.company.tax_calculation_rounding_method == "round_globally"){
            currency_rounding = currency_rounding * 0.00001;
         }
+        var total_excluded = round_pr(price_unit * quantity, currency_rounding);
+        var total_included = total_excluded;
+        var base = total_excluded;
         _(taxes).each(function(tax) {
             tax = self._map_tax_fiscal_position(tax);
             if (tax.amount_type === 'group'){
@@ -1377,11 +1378,14 @@ exports.Orderline = Backbone.Model.extend({
                 }
             }
         });
-        return {taxes: list_taxes, total_excluded: total_excluded, total_included: total_included};
+        return {
+            taxes: list_taxes,
+            total_excluded: round_pr(total_excluded, currency_rounding_bak),
+            total_included: round_pr(total_included, currency_rounding_bak)
+        };
     },
     get_all_prices: function(){
         var price_unit = this.get_unit_price() * (1.0 - (this.get_discount() / 100.0));
-        var taxtotal = 0;
 
         var product =  this.get_product();
         var taxes_ids = product.taxes_id;
@@ -1397,14 +1401,13 @@ exports.Orderline = Backbone.Model.extend({
 
         var all_taxes = this.compute_all(product_taxes, price_unit, this.get_quantity(), this.pos.currency.rounding);
         _(all_taxes.taxes).each(function(tax) {
-            taxtotal += tax.amount;
             taxdetail[tax.id] = tax.amount;
         });
 
         return {
             "priceWithTax": all_taxes.total_included,
             "priceWithoutTax": all_taxes.total_excluded,
-            "tax": taxtotal,
+            "tax": all_taxes.total_included - all_taxes.total_excluded,
             "taxDetails": taxdetail,
         };
     },