[FIX] point_of_sale: html2canvas letterRendering is configurable

The `letterRendering` option from the html2canvas broke the receipt's characters of some localizations/languages such as the arabic one.

With this commit, we can simply override the `htmlToImgLetterRendering()` method from the `PosModel` when needed.

closes odoo/odoo#75777

Related: odoo/enterprise#20520
Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>