[FIX] snailmail: overwrite margins in letters

In order to pass the validation of the snailmail provider, the margins
are required to have nothing but white pixels. To avoid letters being
stuck until their layout is fixed, we fill the margins with white.

closes odoo/odoo#107141

X-original-commit: 9022cba0
Signed-off-by: Florian Daloze (fda) <fda@odoo.com>
Signed-off-by: Louis Baudoux (lba) <lba@odoo.com>